fstools: fix extroot with opkg-installed block-mount (#15550)

The /sbin/block binary now requires and additional shared library, therfore set
LD_LIBRARY_PATH accordingly when invoking it from the temporary overlay.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40526 3c298f89-4303-0410-b956-a3cf2f4a3e73