ramips: switch rt305x subtarget to kernel 5.4

ramips: switch rt305x subtarget to kernel 5.4

RT3x5x seems to work fine with kernel 5.4. Set the default kernel
version to 5.4 to bring this to a broader audience.

Since 4 of 6 targets are on kernel 5.4 now, invert the kernel
version setup logic in Makefile/target.mk files.

Tested on ZyXEL Keenetic.

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>
[invert version setup logic]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>