oxnas: set ubifs parameters for Default profile

oxnas: set ubifs parameters for Default profile

Work-around obsolete mkfs/ubifs code.
Unlike squashfs, ubifs images should be generated per-device so
minimum I/O unit size, logical erase block size, maximum logical erase
block count can be defined for the specific flash-chips parameters.
Also, generating ubinized images is unrelated to rootfs being ubifs or
squashfs, but rather depends on the device supporting UBI or not.
In the meantime and in order to fix an error which currently causes the
image generation on buildbot to fail, work-around by defining
UBIFS_OPTS for the Default profile.
See also
http://buildbot.openwrt.org:8010/builders/oxnas/builds/246/steps/compile_8/logs/stdio

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49068 3c298f89-4303-0410-b956-a3cf2f4a3e73