compat: fix ckmake to skip same base kernels

The ckmake script was allowing testing of compilation
of kernels even if the base kernels were the same. This
fixes it to skip them, trimming down test kernel compilation
down and not caring what old kernels you keep on your
system. This also now prints the RC release correctly.

The get_rel_spec() routine is based on the rel-html
get_rel_spec() routine and rel-html is AGPL but since
I wrote it I relicense a modified version of that
routine here to GPLv2.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>