ramips: mt76x8: enable small_flash feature

ramips: mt76x8: enable small_flash feature

Some of devices in this target have only 8 MiB space and are closing to
borders of usable space. Particularly, TP-Link RE305 v1 already suffers
from this issue[1], where with current partition layout, on release
images, there's not enough space for overlay. So activate small_flash
feature, which will remove some userspace hardening but will gain almost
1 MiB additional flash memory space. Here is small size comparison of
similar device (RE365 v1) with default config + LuCI:

		kernel		rootfs		sysupgrade
current:	2305728		3635044		5964584
small_flash:	1713571		3320132		5047080

1. https://github.com/openwrt/openwrt/issues/14215

Suggested-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>