spl: mmc: raw: Try to load u-boot if Linux image is not found

If CONFIG_SPL_OS_BOOT is enabled and Linux image is not flashed at
RAW_MODE_KERNEL_SECTOR in MMC, spl still assumes that Linux is
available and tries to boot it and hangs. In order to avoid this,
adding a check to verify if parsed image header is of type IH_OS_LINUX.
If it fails then fall back to load u-boot image.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>