x86: spl: Reduce priority of the basic SPL image loader

x86: spl: Reduce priority of the basic SPL image loader

This image loader works on systems where the flash is directly mapped to
the last part of the 32-bit address space. On recent Intel systems (such
as apollolake) this is not the case.

Reduce the priority of this loader so that another one can override it.

While we are here, rename the loader to BOOT_DEVICE_SPI_MMAP since
BOOT_DEVICE_BOARD is not very descriptive.

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