The "proper" vendor prefix for Ubiquiti is "ubnt", this is used in
all targets except ramips and also recommended by the kernel.

This patch adjusts the various board/image/device name variables
accordingly. Since we touch it anyway, this also adds the space
in "EdgeRouter X" as a hyphen to those variables to really make
them consistent with the model name.

While at it, create a real shared definition for the devices in
image/mt7621.mk instead of deriving one device from another.

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