kernel: split kernel version to dedicated files

kernel: split kernel version to dedicated files

Move the kernel versions and hash to dedicated files.
This makes kernel bump quicker and fix some annoying
problem with rebasing when multiple kernel bump are proposed.

Signed-off-by: Ansuel Smith <[email protected]>
[Rebased on top of current master]
Signed-off-by: Rui Salvaterra <[email protected]>
(cherry picked from commit 0765466a42f46f7357e260866a4284ed567bb7ad)
Signed-off-by: Josef Schlehofer <[email protected]>
[Rebased on top of current openwrt-21.02]