[build] Allow DEBUG=... to affect builds of assembler source files

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 PREPROCESS_S	= $(CPP) $(CFLAGS) $(CFLAGS_S) $(OBJ_CFLAGS)
 ASSEMBLE_S	= $(AS) $(ASFLAGS)
 RULE_S		= $(Q)$(PREPROCESS_S) $< | $(ASSEMBLE_S) -o $@
+RULE_S_to_dbg%.o = $(Q)$(PREPROCESS_S) -Ddebug_$(subst -,_,$(OBJECT))=$* $< | $(ASSEMBLE_S) -o $@
 RULE_S_to_s	= $(Q)$(PREPROCESS_S) $< > $@
 
 DEBUG_TARGETS	+= dbg%.o c s

src/include/compiler.h

 /** @defgroup dbg Debugging infrastructure
  * @{
  */
-#ifndef ASSEMBLY
 
 /** @def DBG
  *
  */
 #define DEBUG_SYMBOL PREFIX_OBJECT ( debug_ )
 
+#if DEBUG_SYMBOL == 0
+#define NDEBUG
+#endif
+
+#ifndef ASSEMBLY
+
 /** printf() for debugging
  *
  * This function exists so that the DBG() macros can expand to
 #define DBGCIO_PAUSE( ... )	DBGC_PAUSE_IF	( IO, ##__VA_ARGS__ )
 #define DBGCIO_MORE( ... )	DBGC_MORE_IF	( IO, ##__VA_ARGS__ )
 
-
-#if DEBUG_SYMBOL == 0
-#define NDEBUG
-#endif
-
 #endif /* ASSEMBLY */
 /** @} */