tools: Split apertis-rebase-upstream-kernel

The script is split in a apertis-pull-upstream-kernel and a apertis-rebase-upstream-kernel
script.

The first one only updates the upstream/linux-* branch and the second
one does the actual rebase of the patches on the new upstream.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>