ath79: rename 'engenius' Makefile definitions to 'senao'

ath79: rename 'engenius' Makefile definitions to 'senao'

These recipes and definitions can apply
to devices from other vendors
with PCB boards or SDK produced by Senao
not only the brand Engenius

possible examples:
Extreme Networks, WatchGuard, OpenMesh,
Fortinet, ALLNET, OCEDO, Plasma Cloud, devolo, etc.

so rename all of these items
and move DEVICE_VENDOR from common to generic/tiny.mk

Signed-off-by: Michael Pratt <mcpratt@pm.me>
(cherry picked from commit 70bf4a979c24a5db6f29f6dd691e20eb2345b4d0)