kernel/linux: add alternate download locations

Some longterm versions are not in the usual directory.
Account for these new locations.

Get rid of the mirror location, now that the main kernel site is
(almost) back to normal operations.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""