package/sudo: fix rmdir

When a previous package (e.g. a custom skeleton) or on a rebuild, the
sudoers.d directory may be non-empty, if the user has installed a custom
config file there, so we do not want to remove the directory if it is
not empty; we just want to remove it if it is empty.

Pass --ignore-fail-on-non-empty to rmdir to fix the following error:

rmdir /usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-2/output/target/etc/sudoers.d
rmdir: failed to remove '/usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-2/output/target/etc/sudoers.d': Directory not empty

Fixes:
 - http://autobuild.buildroot.org/results/0cfc61a98760c1d3a98aa0f1e126767881c49abd



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - expand commit log to explain why we don't do rm -rf
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>