omap3_beagle: enable CMD_FS_GENERIC and simplify load of image/ramdisk

CMD_FS_GENERIC allows us to simplify where we load up our image from
either from ext2/fat etc. So, lets use that instead of cumbersome
options we currently use. Sticking with existing conventions,
defaults will be:
ramdisk=ramdisk.gz
bootpart=0:2 (second partition)
bootdir=/boot (/boot in second partition)

This matches with the default behavior, these can be overriden by
env files as needed.

Signed-off-by: Nishanth Menon <nm@ti.com>