Add CONFIG_CMD_EXT4 and CONFIG_CMD_FS_GENERIC

Support reading ext4 partitions and using the generic 'load' command.

Signed-off-by: Kevin Mihelich <kevin@archlinuxarm.org>
Update default environment to use FS_GENERIC load command

Remove the individual attempts to load using ext2 and fat, replace with the
generic load command supporting available filesystem types.

Signed-off-by: Kevin Mihelich <kevin@archlinuxarm.org>