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 <[email protected]>
[invert version setup logic]
Signed-off-by: Adrian Schmutzler <[email protected]>