[build] Formalise mechanism for accessing absolute symbols

In a position-dependent executable, where all addresses are fixed
at link time, we can use the standard technique as documented by
GNU ld to get the value of an absolute symbol, e.g.:

    extern char _my_symbol[];

    printf ( "Absolute symbol value is %x\n", ( ( int ) _my_symbol ) );

This technique may not work in a position-independent executable.
When dynamic relocations are applied, the runtime addresses will no
longer be equal to the link-time addresses.  If the code to obtain the
address of _my_symbol uses PC-relative addressing, then it will
calculate the runtime "address" of the absolute symbol, which will no
longer be equal the the link-time "address" (i.e. the correct value)
of the absolute symbol.

Define macros ABS_SYMBOL(), ABS_VALUE_INIT(), and ABS_VALUE() that
provide access to the correct values of absolute symbols even in
position-independent code, and use these macros wherever absolute
symbols are accessed.

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

src/image/embedded.c

@@ -16,7 +16,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #undef EMBED
 #define EMBED( _index, _path, _name )					\
 	extern char embedded_image_ ## _index ## _data[];		\
-	extern char embedded_image_ ## _index ## _len[];		\
+	extern size_t ABS_SYMBOL ( embedded_image_ ## _index ## _len );	\
 	__asm__ ( ".section \".data\", \"aw\", " PROGBITS "\n\t"	\
 		  "\nembedded_image_" #_index "_data:\n\t"		\
 		  ".incbin \"" _path "\"\n\t"				\
@@ -34,7 +34,7 @@ EMBED_ALL
 	.name = _name,							\
 	.flags = ( IMAGE_STATIC | IMAGE_STATIC_NAME ),			\
 	.rwdata = embedded_image_ ## _index ## _data,			\
-	.len = ( size_t ) embedded_image_ ## _index ## _len,		\
+	.len = ABS_VALUE_INIT ( embedded_image_ ## _index ## _len ),	\
 },
 static struct image embedded_images[] = {
 	EMBED_ALL