ramips: move edimax images to the new build code

Images installed via TFTP recovery or the Edimax webinterface of the
3g-6200n(l) are writting with the edimax header to flash.

Use only one image type for these boards. The migration to the
factory only images need to be done via TFTP recovery.

Use the same start address for the 3g-6200n(l) factory images as the
stock firmware images.

Thanks to Jan Dostrasil for reporting all the issues of the 3g-6200nl
and the patient testing of all changes.

Signed-off-by: Mathias Kresin <dev@kresin.me>