ARM: omap3: evm: Add kernel image loading from UBIFS and EXT4

This commit adds UBIFS_NAND to BOOT_TARGET_DEVICES. This will
allow the kernel zImage to be loaded from '/boot/zImage' in UBIFS
(ubi0:rootfs).

Additionally update the *_MMC devices to also load kernel image from
the MMC 0:2 EXT4 file system.

DISTRO_DEFAULTS Setup

=====================

[primary] Check MMC 0:1 for /extlinux/extlinux.conf and boot
[fallback 1] Check MMC 0:2 /boot/zImage and run mmcbootz
[fallback 2] Check MMC 0:2 /boot/uImage and run mmcboot
[fallback 3] Check NAND UBIFS /boot/zImage and run nandbootubifs
[fallback 4] Check NAND partitions and run nandboot

The following "extlinux.conf" can be used to load images in the
top-level of the MMC 0:1 FAT partition.

[MMC(0:1)/extlinux/extlinux.conf]
---8<-------------------------------------------------------------------
default omap3-evm
label omap3-evm
	kernel /zImage
	fdt /omap3-evm.dtb
	append console=ttyO0,115200n8 root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait
---8<-------------------------------------------------------------------

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>