opkg-keyrings: check if opkg-key exists before run postinst

opkg-keyrings: check if opkg-key exists before run postinst

By default, the opkg-key command is not included in pokg package because
it is only installed when gpg support is enabled. We'd better check if
it exists before run 'opkg-key populate' in pkg_postinst.

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>