kernel: use older kernel for explicitly setting dependencies

kernel: use older kernel for explicitly setting dependencies

It is generally more desirable to use older kernel versions for
dependencies, as this will require less changes when newer kernels
are added (they will by default select the newer packages).

Since we currently only have two kernels (4.14 and 4.19) in master,
this patch applies this logic by converting all LINUX_4_19 symbols
to their inverted LINUX_4_14 equivalents.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>