Small journal size can lead to problems if nand flash is too big. By default
ubifs will use ~12% of volume size or a maximum of 8MiB.

Signed-off-by: Luka Perkov <luka@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38799 3c298f89-4303-0410-b956-a3cf2f4a3e73