buildroot

opkg: add missing dependency on libarchive

opkg requires libarchive. This was expressed in the Config.in file with a
select, but not in the .mk file.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>