[build] Fix the REQUIRE_SYMBOL mechanism

At some point in the past few years, binutils became more aggressive
at removing unused symbols.  To function as a symbol requirement, a
relocation record must now be in a section marked with @progbits and
must not be in a section which gets discarded during the link (either
via --gc-sections or via /DISCARD/).

Update REQUIRE_SYMBOL() to generate relocation records meeting these
criteria.  To minimise the impact upon the final binary size, we use
existing symbols (specified via the REQUIRING_SYMBOL() macro) as the
relocation targets where possible.  We use R_386_NONE or R_X86_64_NONE
relocation types to prevent any actual unwanted relocation taking
place.  Where no suitable symbol exists for REQUIRING_SYMBOL() (such
as in config.c), the macro PROVIDE_REQUIRING_SYMBOL() can be used to
generate a one-byte-long symbol to act as the relocation target.

If there are versions of binutils for which this approach fails, then
the fallback will probably involve killing off REQUEST_SYMBOL(),
redefining REQUIRE_SYMBOL() to use the current definition of
REQUEST_SYMBOL(), and postprocessing the linked ELF file with
something along the lines of "nm -u | wc -l" to check that there are
no undefined symbols remaining.

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

src/config/config.c

@@ -44,6 +44,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  * in the final iPXE executable built.
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in all requested console types
  *

src/config/config_ethernet.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Ethernet-specific protocols
  */

src/config/config_fc.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Fibre Channel-specific commands
  *

src/config/config_infiniband.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in Infiniband-specific protocols
  */

src/config/config_net80211.c

@@ -25,6 +25,8 @@ FILE_LICENCE ( GPL2_OR_LATER );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in 802.11-specific commands
  *

src/config/config_romprefix.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Provide UNDI loader if PXE stack is requested
  *

src/config/config_route.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in routing management for relevant protocols
  *

src/config/config_usb.c

@@ -29,6 +29,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 
+PROVIDE_REQUIRING_SYMBOL();
+
 /*
  * Drag in USB controllers
  */