ARM: mx6: Enable MMC and SATA extfs boot support

Enable support for booting U-Boot image from ext filesystem when either
SD/MMC or SATA support is compiled into the SPL. This will allow easy
transition from loading U-Boot image from ad-hoc offset on the card to
loading U-Boot image from the filesystem. VFAT support is intently not
enabled. The boot order is tweaked so that raw is tested first and if
the raw has no signature, FS boot is attempted.

To install just the SPL on i.MX6 board, perform the following operation
 $ dd if=SPL of=/dev/sdX seek=2 bs=512
To install the U-Boot image, copy u-boot.img to the first partition of
the SD/MMC/SATA drive. The partition must be formated to extfs.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@konsulko.com>