[build] Fix %.licence build target

Our use of --gc-sections causes the linker to discard the symbols
defined by FILE_LICENCE(), meaning that the resulting licence
determination is incomplete.

We must use the KEEP() directive in the linker script to force the
linker to not discard the licence symbols.  Using KEEP(*(COMMON))
would be undesirable, since there are some symbols in COMMON which we
may wish to discard.

Fix by placing symbols defined by PROVIDE_SYMBOL() (which is used by
FILE_LICENCE()) into a special ".provided" section, which we then mark
with KEEP().  All such symbols are zero-length, so there is no cost in
terms of the final binary size.

Since the symbols are no longer in COMMON, the linker will reject
symbols with the same name coming from multiple objects.  We therefore
append the object name to the licence symbol, to ensure that it is
unique.

Reported-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 # Get licensing verdict for the specified target
 #
 define licensable_deps_list
-	$(filter-out config/local/%.h,$(call deps_list,$(1)))
+	$(filter-out config/local/%.h,\
+	  $(filter-out $(BIN)/.%.list,\
+	    $(call deps_list,$(1))))
 endef
 define unlicensed_deps_list
 	$(shell grep -L FILE_LICENCE $(call licensable_deps_list,$(1)))
 endef
 define licence_list
-	$(patsubst __licence_%,%,\
-	  $(filter __licence_%,$(shell $(NM) $(1) | cut -d" " -f3)))
+	$(sort $(foreach LICENCE,\
+		 $(filter __licence__%,$(shell $(NM) $(1) | cut -d" " -f3)),\
+		 $(word 2,$(subst __, ,$(LICENCE)))))
 endef
+$(BIN)/%.licence_list : $(BIN)/%.tmp
+	$(Q)$(ECHO) $(call licence_list,$<)
 $(BIN)/%.licence : $(BIN)/%.tmp
 	$(QM)$(ECHO) "  [LICENCE] $@"
 	$(Q)$(if $(strip $(call unlicensed_deps_list,$<)),\

src/arch/i386/scripts/i386-kir.lds

 	*(.data)
 	*(.data.*)
 	KEEP(*(SORT(.tbl.*)))	/* Various tables.  See include/tables.h */
+	KEEP(*(.provided))
+	KEEP(*(.provided.*))
 	_edata16_progbits = .;
     }
     .bss16 : AT ( _data16_load_offset + __bss16 ) {

src/arch/i386/scripts/i386.lds

 	*(.data)
 	*(.data.*)
 	KEEP(*(SORT(.tbl.*)))	/* Various tables.  See include/tables.h */
+	KEEP(*(.provided))
+	KEEP(*(.provided.*))
 	_mtextdata = .;
     } .bss.textdata (NOLOAD) : AT ( _end_lma ) {
 	*(.bss)

src/arch/i386/scripts/linux.lds

 		*(.data)
 		*(.data.*)
 		KEEP(*(SORT(.tbl.*)))
+		KEEP(*(.provided))
+		KEEP(*(.provided.*))
 		_edata = .;
 	}
 

src/arch/x86/scripts/efi.lds

 	*(.data)
 	*(.data.*)
 	KEEP(*(SORT(.tbl.*)))	/* Various tables.  See include/tables.h */
+	KEEP(*(.provided))
+	KEEP(*(.provided.*))
 	_edata = .;
     }
 

src/arch/x86_64/scripts/linux.lds

 		*(.data)
 		*(.data.*)
 		KEEP(*(SORT(.tbl.*)))
+		KEEP(*(.provided))
+		KEEP(*(.provided.*))
 		_edata = .;
 	}
 

src/include/compiler.h

 /** Provide a symbol within this object file */
 #ifdef ASSEMBLY
 #define PROVIDE_SYMBOL( _sym )				\
+	.section ".provided", "a", @nobits ;		\
+	.hidden _sym ;					\
 	.globl	_sym ;					\
-	.comm	_sym, 0
+	_sym: ;						\
+	.previous
 #else /* ASSEMBLY */
 #define PROVIDE_SYMBOL( _sym )				\
-	extern char _sym[];				\
-	char _sym[0]
+	char _sym[0]					\
+	  __attribute__ (( section ( ".provided" ) ))
 #endif /* ASSEMBLY */
 
 /** Require a symbol within this object file
  * be in the public domain.
  */
 #define FILE_LICENCE_PUBLIC_DOMAIN \
-	PROVIDE_SYMBOL ( __licence_public_domain )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__public_domain__ ) )
 
 /** Declare a file as being under version 2 (or later) of the GNU GPL
  *
  * (at your option) any later version".
  */
 #define FILE_LICENCE_GPL2_OR_LATER \
-	PROVIDE_SYMBOL ( __licence_gpl2_or_later )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__gpl2_or_later__ ) )
 
 /** Declare a file as being under version 2 of the GNU GPL
  *
  * "or, at your option, any later version" clause.
  */
 #define FILE_LICENCE_GPL2_ONLY \
-	PROVIDE_SYMBOL ( __licence_gpl2_only )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__gpl2_only__ ) )
 
 /** Declare a file as being under any version of the GNU GPL
  *
  * version ever published by the Free Software Foundation".
  */
 #define FILE_LICENCE_GPL_ANY \
-	PROVIDE_SYMBOL ( __licence_gpl_any )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__gpl_any__ ) )
 
 /** Declare a file as being under the three-clause BSD licence
  *
  * functionally equivalent to the standard three-clause BSD licence.
  */
 #define FILE_LICENCE_BSD3 \
-	PROVIDE_SYMBOL ( __licence_bsd3 )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__bsd3__ ) )
 
 /** Declare a file as being under the two-clause BSD licence
  *
  * functionally equivalent to the standard two-clause BSD licence.
  */
 #define FILE_LICENCE_BSD2 \
-	PROVIDE_SYMBOL ( __licence_bsd2 )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__bsd2__ ) )
 
 /** Declare a file as being under the one-clause MIT-style licence
  *
  * permission notice appear in all copies.
  */
 #define FILE_LICENCE_MIT \
-	PROVIDE_SYMBOL ( __licence_mit )
+	PROVIDE_SYMBOL ( PREFIX_OBJECT ( __licence__mit__ ) )
 
 /** Declare a particular licence as applying to a file */
 #define FILE_LICENCE( _licence ) FILE_LICENCE_ ## _licence