x86: Rename coreboot-x86 to coreboot

We only use coreboot as a target on x86 platforms, since on ARM platforms
U-Boot always runs as the primary boot loader. Rename the coreboot-x86
platform to reflect this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

board/coreboot/coreboot/MAINTAINERS

@@ -3,4 +3,4 @@ M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
 F:	board/coreboot/coreboot/
 F:	include/configs/chromebook_link.h
-F:	configs/coreboot-x86_defconfig
+F:	configs/coreboot_defconfig

doc/README.x86

@@ -46,7 +46,7 @@ Build Instructions for U-Boot as coreboot payload
 Building U-Boot as a coreboot payload is just like building U-Boot for targets
 on other architectures, like below:
 
-$ make coreboot-x86_defconfig
+$ make coreboot_defconfig
 $ make all
 
 Note this default configuration will build a U-Boot payload for the QEMU board.