ar71xx: add support for TP-Link TL-WDR6500 v2

Signed-off-by: Weijie Gao <hackpascal@gmail.com>

This patch adds support for TP-Link TL-WDR6500 v2.

The firmware has a U-Boot header for kernel, and a TP-LINK v2 header for
the whole firmware, so I have to create a new firmware creation method.

SVN-Revision: 46663