[linker] Add mechanism for subsystem-dependent configuration options

It is often the case that some module of gPXE is only relevant if the
subsystem it depends on is already being included. For instance,
commands to manage wireless interfaces are quite useless if no
compiled-in driver has pulled in the wireless networking stack. There
may be a user-modifiable configuration options for these dependent
modules, but even if enabled, they should not be included when they
would be useless.

Solve this by allowing the creation of config_subsystem.c, for
configuration directives like those in the global config.c that should
only be considered when subsystem.c is included in the final gPXE
build.

For consistency, move core/config.c to the config/ directory, where
the other config_subsystem.c files will eventually reside.

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/Makefile

 SRCDIRS		+= hci hci/commands hci/tui
 SRCDIRS		+= hci/mucurses hci/mucurses/widgets
 SRCDIRS		+= usr
+SRCDIRS		+= config
 
 # NON_AUTO_SRCS lists files that are excluded from the normal
 # automatic build system.

src/core/config.c → src/config/config.c

 #include <config/general.h>
 #include <config/console.h>
 
+/** @file
+ *
+ * Configuration options
+ *
+ * This file contains macros that pull various objects into the link
+ * based on definitions in configuration header files. Ideally it
+ * should be the only place in gPXE where one might need to use #ifdef
+ * for compile-time options.
+ *
+ * In the fairly common case where an object should only be considered
+ * for inclusion if the subsystem it depends on is present, its
+ * configuration macros should be placed in a file named
+ * <tt>config_<i>subsystem</i>.c</tt>, where @e subsystem is the
+ * object basename of the main source file for that subsystem. The
+ * build system will pull in that file if @c subsystem.c is included
+ * in the final gPXE executable built.
+ */
+
 /*
  * Build ID string calculations
  *

src/include/compiler.h

 
 #define PREFIX_OBJECT( _prefix ) _C2 ( _prefix, OBJECT )
 #define OBJECT_SYMBOL PREFIX_OBJECT ( obj_ )
+#define REQUEST_EXPANDED( _sym ) REQUEST_SYMBOL ( _sym )
+#define CONFIG_SYMBOL PREFIX_OBJECT ( obj_config_ )
 
 /** Always provide the symbol for the current object (defined by -DOBJECT) */
 PROVIDE_SYMBOL ( OBJECT_SYMBOL );
 
+/** Pull in an object-specific configuration file if available */
+REQUEST_EXPANDED ( CONFIG_SYMBOL );
+
 /** Explicitly require another object */
 #define REQUIRE_OBJECT( _obj ) REQUIRE_SYMBOL ( obj_ ## _obj )