Since all x86 boards use spi flash as its bootloader storage media,
it makes sense to make CONFIG_ENV_IS_IN_SPI_FLASH a common option.
So far only minnowmax board does not support it so undefine it in
its board configuration file.

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