So far, image/device/board names for Mikrotik devices in mt7621 have
been used quite inconsistently.

This patch harmonizes the naming scheme by applying the same style
as used lately in ath79, i.e. using "RouterBOARD" as separate word
in the model name (instead of RB prefix for the number) and deriving
the board/device name from that (= make lower case and replace spaces
by hyphens).

This style has already been used for most the model/DEVICE_MODEL
variables in mt7621, so this is essentially just adjusting the remaining
variables to that.

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