The target uses 5.4 as default kernel since 06/2020.
Kernel 4.19 support is not really maintained anymore, it does not
seem to be needed and upcoming changes (mainly DSA) will break
backward-compatibility anyway.
Thus, make maintaining of old stuff and reviewing of new stuff
easier by removing support for kernel 4.19.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>