ramips: improve common definition for Netgear R6xxx in mt7621.mk

ramips: improve common definition for Netgear R6xxx in mt7621.mk

This patch renames and reassembles the common definition for
Netgear R6xxx devices in mt7621.mk. The following goals should be
achieved:
- Give the node a more generic name instead of adding devices to it
- Use the common definition in the (less) similar R6220 node
- Include/exclude settings into the common definition so the common
  node contains the common definitions
- Prepare for support of R6700 v2

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