Fix the error message for romstage when .bss or .data are non-zero

The error message from romstage is annoying and misleading:
"Do not use global variables in romstage"

Because it can occur even when global variables are not used
in some circumstances, but also because it gives you only a rough
idea where to look. This change sucks but sucks less. We still don't
know which file the problem is in but at least we know if it is data
or bss.

Replace the error message with something that provides more information
and less guessing on the part of the script:
".bss is non-zero size in romstage which is not allowed -- global variable?"
or
".data is non-zero size in romstage which is not allowed -- global variable?"

To test: build coreboot as normal. It builds.
Add
char d[32];
to romstage.c and get the first error message; add
int x = 32;
to romstage.c and get the second.

Change-Id: I300ec05bdb4b30d7ef3f5112e6cc09b1fafe8263
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/1160
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/arch/x86/init/romstage.ld

@@ -53,5 +53,6 @@ SECTIONS
 	}
 
 	_bogus = ASSERT((SIZEOF(.car.data) <= CONFIG_DCACHE_RAM_SIZE), "Cache as RAM area is too full");
-	_bogus = ASSERT((SIZEOF(.bss) + SIZEOF(.data)) == 0 || CONFIG_CPU_AMD_AGESA, "Do not use global variables in romstage");
+	_bogus = ASSERT(SIZEOF(.bss) == 0 || CONFIG_CPU_AMD_AGESA, ".bss is non-zero size in romstage which is not allowed -- global variable?");
+	_bogus = ASSERT(SIZEOF(.data) == 0 || CONFIG_CPU_AMD_AGESA, ".data is non-zero size in romstage which is not allowed -- global variable?");
 }