ARM: omap3: evm: Fix distro bootcmd UBIFS and MMC support

The omap3_evm board does not boot when commit:
    a47ca2cf ("ARM: omap3: evm: Add kernel image loading from UBIFS and EXT4")
is applied after commit:
    3dde8f20 ("Merge git://git.denx.de/u-boot-mmc"

)

This commit reduces the CONFIG_EXTRA_ENV_SETTINGS size and better
leverages the existing distro bootcmd infrastructure.

- Use updated UBIFS support from config_distro_bootcmd.h
- Use LEGACY_MMC naming found in am335x_evm.h and ti_omap4_common.h
- Remove extra environment content that is no longer needed

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

[MMC(0:2)/boot/extlinux/extlinux.conf]
---8<-------------------------------------------------------------------
default omap3-evm-mmc-ext4
label omap3-evm-mmc-ext4
	kernel /boot/zImage
	fdt /boot/omap3-evm.dtb
	append console=ttyO0,115200n8 root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait
---8<-------------------------------------------------------------------

[NAND(ubi0:rootfs)/boot/extlinux/extlinux.conf]
---8<-------------------------------------------------------------------
default omap3-evm-nand-ubifs
label omap3-evm-nand-ubifs
	kernel /boot/zImage
	fdt /boot/omap3-evm.dtb
	append console=ttyO0,115200n8 root=ubi0:rootfs rw ubi.mtd=rootfs noinitrd rootfstype=ubifs rootwait
---8<-------------------------------------------------------------------

Fixes: a47ca2cf ("ARM: omap3: evm: Add kernel image loading from UBIFS and EXT4")
Signed-off-by: Derald D. Woods <woods.technical@gmail.com>