src: Fix the warning "type 'hex' are always defined"

This is spotted using "./util/lint/kconfig_lint"
While at it, do the check in C and not the preprocessor.

Change-Id: Icfda267936a23d9d14832116d67571f42f685906
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/31050
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/arch/x86/include/arch/memlayout.h

@@ -23,7 +23,7 @@
 #define ARCH_STAGE_HAS_BSS_SECTION 0
 #endif
 
-#if !defined(CONFIG_RAMTOP) || !CONFIG_RAMTOP
+#if (CONFIG_RAMTOP == 0)
 # error "CONFIG_RAMTOP not configured"
 #endif