opkg-utils: Do not use --ignore-fail-on-non-empty with rmdir

opkg-utils: Do not use --ignore-fail-on-non-empty with rmdir

--ignore-fail-on-non-empty is coreutils specific, and you
are not always going to have coreutils on target systems
especially small ones. They will use the busybox applet which
does not support --ignore-fail-on-non-empty, use pipe and true
to ignore the errorcode from rmdir instead

Fixes upgrade errors on target e.g.

rmdir: unrecognized option '--ignore-fail-on-non-empty'
BusyBox v1.24.1 (2016-12-20 10:41:39 PST) multi-call binary.

Usage: rmdir [OPTIONS] DIRECTORY...
To remove package debris, try `opkg remove update-alternatives-opkg`.
To re-attempt the install, try `opkg install update-alternatives-opkg`.
Collected errors:
 * pkg_run_script: package "update-alternatives-opkg" postrm script returned status 1.
 * postrm_upgrade_old_pkg: postrm script for package "update-alternatives-opkg" failed

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>