As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

This also takes us past the OpenWrt 15.05 final release.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.

These packages are included by OpenWrt by default on this target and don't
need to be specified manually.

Original patch by @MPW1412