ath79: split base-files into subtargets

While most of the target's contents are split into subtargets, the
base-files are maintained for the target as a whole.

However, OpenWrt already implements a mechanism that will use (and
even prefer) files in the subtargets' directories. This can be
exploited to make several scripts subtarget-specific and thus save
some space (especially helpful for the tiny devices).

The only script remaining in parent base-files is
/etc/hotplug.d/ieee80211/00-wifi-migration, everything else is
moved/split.

Note that this will increase overall code lines, but reduce code
per subtarget.

base-files ipk size reduction:
master (generic)   49135 B
split (generic)    48533 B (- 0.6 kiB)
split (tiny)       43337 B (- 5.7 kiB)
split (nand)       44423 B (- 4.6 kiB)

Tested on TL-WR1043ND v4 (generic) and TL-WR841N v12 (tiny).

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