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 <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>

arch/x86/cpu/coreboot/Makefile

@@ -13,10 +13,10 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-obj-$(CONFIG_SYS_COREBOOT) += car.o
-obj-$(CONFIG_SYS_COREBOOT) += coreboot.o
-obj-$(CONFIG_SYS_COREBOOT) += tables.o
-obj-$(CONFIG_SYS_COREBOOT) += ipchecksum.o
-obj-$(CONFIG_SYS_COREBOOT) += sdram.o
-obj-$(CONFIG_SYS_COREBOOT) += timestamp.o
+obj-y += car.o
+obj-y += coreboot.o
+obj-y += tables.o
+obj-y += ipchecksum.o
+obj-y += sdram.o
+obj-y += timestamp.o
 obj-$(CONFIG_PCI) += pci.o