The postinst script is sourced during image build, which causes the
follow failure:
/home/stijn/Development/OpenWrt/openwrt/build_dir/target-x86_64_musl/root-x86/etc/init.d/lxc-auto: line 3: /lib/functions.sh: No such file or directory
postinst script ./usr/lib/opkg/info/lxc-auto.postinst has failed with exit code 1

Sourcing /lib/functions.sh is not needed, as /etc/rc.common does so
already. Unfortunately removing that line from the init script is not
enough to fix the problem. The postinst script should also check
IPKG_INSTROOT. As these two changes are unrelated, they should go in
separate commits, and the solution to the image build problem is to
revert the commit that introduced the breakage.

This reverts commit 2cde10b950.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
(cherry picked from commit 039912dec5)