x86: use CONFIG_SYS_COREBOOT to descend into coreboot/ directory

x86: use CONFIG_SYS_COREBOOT to descend into coreboot/ directory

The references of CONFIG_SYS_COREBOOT in arch/x86/cpu/coreboot/Makefile
are redundant because the build system descends into the directory
only when CONFIG_SYS_COREBOOT is defined.

Signed-off-by: Masahiro Yamada <[email protected]>
Cc: Simon Glass <[email protected]>
Acked-by: Simon Glass <[email protected]>