Kconfig: Add an option to skip adding a cbfs bootblock on x86

Some targets don't need this as the bootblock is loaded differently.

Change-Id: Ia42448f7e9dd0635c72857fbc1fab54508932721
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63377
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/lib/master_header_pointer.c

@@ -5,7 +5,7 @@
 #include <fmap_config.h>
 #include <stdint.h>
 
-#if ENV_X86
+#if CONFIG(BOOTBLOCK_IN_CBFS)
 __attribute__((used, __section__(".header_pointer")))
 #endif