x86: coreboot: Make SYS_CONFIG_NAME and DEFAULT_DEVICE_TREE configurable

In theory U-Boot built for coreboot is supposed to run as a payload
to be loaded by coreboot on every board that coreboot supports.
The U-Boot build process uses SYS_CONFIG_NAME and DEFAULT_DEVICE_TREE
which are hardcoded in board defconfig and Kconfig files. For better
support of coreboot, we want to make these two options configurable
so that we can easily change them during 'make menuconfig' so that
the generated U-Boot image for coreboot is board configuration aware.

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

board/coreboot/coreboot/Kconfig

@@ -9,7 +9,20 @@ config SYS_VENDOR
 config SYS_SOC
 	default "coreboot"
 
+comment "coreboot-specific options"
+
 config SYS_CONFIG_NAME
+	string "Board configuration file"
 	default "coreboot"
+	help
+	  This option selects the board configuration file in include/configs/
+	  directory to be used to build U-Boot for coreboot.
+
+config DEFAULT_DEVICE_TREE
+	string "Board Device Tree Source (dts) file"
+	default "link"
+	help
+	  This option selects the board Device Tree Source (dts) file in
+	  arch/x86/dts/ directory to be used to build U-Boot for coreboot.
 
 endif