linux: add shorter shortcuts

We only have one Linux kernel package, and "linux26-" is an anachronism
in today's world.  So add useful "linux-%" shortcuts to the "linux26-%".

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>