ath79: move ubnt-xm to tiny

ath79: move ubnt-xm to tiny

ath79 has was bumped to 5.10. With this, as with every kernel change,
the kernel has become larger. However, although the kernel gets bigger,
there are still enough flash resources. But the RAM reaches its capacity
limits. The tiny image comes with fewer kernel flags enabled and
fewer daemons.

Improves: 15aa53d7ee65 ("ath79: switch to Kernel 5.10")

Tested-by: Robert Foss <me@robertfoss.se>
Signed-off-by: Nick Hainke <vincent@systemli.org>