getent: look for glibc's getent in more places

External toolchains don't always install the executable in /usr/bin -
God knows why. So check in a few more places.

We can use make's $(wildcard ...) here, because the variable only gets
expanded when the install step is executed, and by that time the
STAGING_DIR has already been installed.

Probably fixes most getent autobuilder errors, at least if fixes
http://autobuild.buildroot.net/results/615b9d17f713b4a53192efb00188560a76b9efa3/



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>