ramips-mt76x8: add support for TP-Link Archer C50 v4 (#1641)

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