ramips: switch rt288x subtarget to kernel 5.4

ramips: switch rt288x subtarget to kernel 5.4

The sbutarget has testing support for kernel 5.4 for quite a while
and builds fine, however, only one devices there is > 4 MiB.

Since it's unlikely to get a Tested-by for that device, and the other
ralink subtargets appear to be working with 5.4 so far, let's set
this target to 5.4 by default as well.

That way, even if the device happens to break, we'll still have at
least usable SDK and IB for people to use.

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