ar71xx: Move some targets to tiny

ar71xx: Move some targets to tiny

Recent changes on ar71xx (switch to 4.14, memory compaction, ...) cause
an increase in kernel size, making it too big for some devices.

Move these devices to the tiny target, where kernel symbols and
optimization for speed are disabled, reducing the kernel size.

Devices:
- EnGenius ENS202EXT
- OCEDO Koala

Compile-tested targets:

- ar71xx->generic->default
- ar71xx->smallFlash->Default

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(backported from 5f249333090741ecba190855b13c87fd93ca2520)