base-files: fix user creation on sysupgrade with few opkg control files

If only a single opkg control file exists (which can happen with
CONFIG_CLEAN_IPKG), grep would not print the file name by default. Instead
of forcing it using -H, we just switch to -l (print only file names) and
get rid of the cut.

Add -s to suppress an error message when no control files exist.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>