ath79: reorganize common image definitions for Netgear

ath79: reorganize common image definitions for Netgear

Netgear currently has a special definition for tiny devices, which
is only used by two devices. Despite, it sets ups the IMAGE/default
definition individually for all devices, although there is actually
only one exception.

This merges the common parts into a single netgear_generic definition
(in contrast to netgear_ath79_nand), and adjusts the individual
definitions accordingly.

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