The target has testing support for kernel 5.4 for quite a while,
compiles fine for all devices, and has been run-tested on Asus
RT-N56U successfully.
Let's set it to kernel 5.4 by default to increase the audience
before an 20.xx stable branch.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Eneas U de Queiroz <cotequeiroz@gmail.com> [Asus RT-N56U]
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