tools/firmware-utils: add header version 2 support for mktplinkfw

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

This patch adds header version 2 option for mktplinkfw.

The version 2 header is used for AR/QCA firmwares and is not the same as
the header generated by mktplinkfw2.
Instead, it is nearly the same as version 1 header except for the header
version and the RSA signature.

The header version 2 support is used for newer TP-Link routers which have
only a 64kb bootloader part, e.g. TL-WDR6500 v2.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46661 3c298f89-4303-0410-b956-a3cf2f4a3e73