spl: Convert SYS_MMCSD_RAW_MODE_KERNEL_SECTOR to Kconfig

Falcon mode is very useful in improving boot speed. A question that
Falcon mode asks is "Where do I look for the kernel". With MMC boot
media, the correct answer is CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR.
The scope of this patch is to move this to Kconfig.

It is possible for a system to support Falcon mode from NOR but not
MMC. In that case, mmc_load_image_raw_os() would not be used. To
address this, conditionally compile mmc_load_image_raw_os() when
SPL_FALCON_BOOT_MMCSD, instead of SPL_OS_BOOT.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Move spl_start_uboot to its own guard in spl_mmc.c, rerun migration]
Signed-off-by: Tom Rini <trini@konsulko.com>