Move global variable check to Makefile

Our linker script for romstage checks for global variables and
makes the build fail if there are any (on non-AMD systems).
This is great, but having the build fail without any indication
which variables are global is not very useful.

Moving the check to the Makefile allows us to let the linking stage
succeed and reveil which variable names end up in the data and bss
sections of the binary.

To test, add "int foo;" as the first line in src/mainboard/samsung/lumpy/romstage.c
and build coreboot for Lumpy. See the build break the following
way:

    LINK       cbfs/fallback/romstage_null.debug
    Forbidden global variables in romstage:
    00006a84 B foo

Change-Id: I3c8780888f46a6577ffd36bcea317997b4f84f6f
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/1692
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>

src/arch/x86/Makefile.inc

@@ -341,6 +341,10 @@ ifeq ($(CONFIG_COMPILER_LLVM_CLANG),y)
 else
 	$(CC) -nostdlib -nostartfiles -static -o $@ -L$(obj) -T $(objgenerated)/romstage_null.ld $(romstage-objs)
 endif
+	$(NM) $@ | grep -q " [DdBb] "; if [ $$? -eq 0 ]; then \
+		echo "Forbidden global variables in romstage:"; \
+		$(NM) $@ | grep " [DdBb] "; test "$(CONFIG_CPU_AMD_AGESA)" == y; \
+		else true; fi
 
 $(objcbfs)/romstage_xip.debug: $$(romstage-objs) $(objgenerated)/romstage_xip.ld
 	@printf "    LINK       $(subst $(obj)/,,$(@))\n"

src/arch/x86/init/romstage.ld

@@ -53,6 +53,4 @@ SECTIONS
 	}
 
 	_bogus = ASSERT((SIZEOF(.car.data) <= CONFIG_DCACHE_RAM_SIZE), "Cache as RAM area is too full");
-	_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?");
 }