Revert "SW: drop ifenslave from GRML_SMALL and GRML_FULL"

This reverts commit 46732edb357fda04f5778a4dcb6af1224d002cf3.

ifenslave is back in Debian/testing, while it might not be really needed
anymore (given that it no longer provides /sbin/ifenslave and "the
/sbin/ip command from the iproute2 package supports creating bonding masters
and enslaving other interfaces to it"), its hook script still might be
relevant for someone, so let's ship it for the time being.