x86: coreboot: Make SYS_CONFIG_NAME and DEFAULT_DEVICE_TREE configurable

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>