[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

@@ -73,6 +73,7 @@ SRCDIRS		+= crypto crypto/axtls crypto/matrixssl
 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.