generic/3.10: reduce number of ifdefs in the rootfs split code

Use the config_enabled() macro where it is possible.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37980 3c298f89-4303-0410-b956-a3cf2f4a3e73