Fix extroot by UUID

Fix extroot by UUID

Extroot works fine when the target device is specified by a path. It fails
however if the device is specified by UUID (the target partition gets mounted
much later by hotplug hooks). This is because the blkid command is no longer
compiled into BusyBox (since changeset [1]) so it's unavailable for the
preinit phase.

The closest bug report I was able to find is [2], although the reporting person
mentions that /tmp/overlay-disabled showed up which wasn't there in my case.

This patch sets PATH and LD_LIBRARY_PATH environment variables so that the
blkid command installed on the target device can be used by that particular
preinit script.

[1] https://dev.openwrt.org/changeset/26245
[2] https://dev.openwrt.org/ticket/10653

Signed-off-by: Jaroslaw Swierczynski <jarek1701@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32567 3c298f89-4303-0410-b956-a3cf2f4a3e73