purge-locales: Handle empty locale directories better

purge-locales: Handle empty locale directories better

If a locale directory is empty, shell code like "for langdir in
$$dir/*;" will loop once with langdir set to "path/to/dir/*", rather
than not looping at all, which would obviously be the desired
behavior.

Then "grep -qx $${langdir##*/}" ungoes two shell expansions (how?)
that transform the expression from "${langdir##*/}" to "*" to "list of
all files in buildroot root dir".  Which is most certainly not what
this command was supposed to do.

If one of those files happens to be an 8GB flash image, grep consumes
all available memory and crashes trying to search it.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>