ramips: harmonize device vendor Zbtlink

ramips: harmonize device vendor Zbtlink

Spelling of Zbtlink varies across image definitions and DTS files.

This patch uses Zbtlink consistently and also updates the model
in DTS files to contain the vendor in all cases.

This patch is cosmetical, as there should be no dependencies on
device model name in ramips anymore.

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