x86: kconfig: Let board select SPI flash

Only a specific type of SPI flash exists on a board, having board
Kconfig to select the SPI flash seems to make more sense. Other
flash types are not necessary except coreboot, which implies all
available flash drivers there.

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