This backports the TP-Link Archer C50 v4.

We are dropping the following upstream commits. They add support for the
TP-Link recovery-flag which enabled the web-recovery. As they are not
needed for the router to work, we drop them for now.

28cd2ca base-files: sysupgrade: support additional mtd options
1e06482 mtd: add logic for TP-Link ramips recovery magic

This commit removes the broken flag from all devices in the mt76x8
subtarget.

The stability of the mt76 driver for the mt7628 and mt7612 has greatly
improved in the last half-year. It might be still behind ath9k and
ath10k but it is suitable for daily use.

This affects the following devices:

 - GL.iNet MT300N v2
 - TP-Link Archer C50 v3
 - TP-Link TL-WR841 v13

The driver has stablized enough to officially support ramips-mt76x8 devices
again. At least remove BROKEN for VoCore 2 (as it was supported before).

Signed-off-by: Martin Weinelt <martin@darmstadt.freifunk.net>