ar71xx/image: remove duplicated rootfs in new tplink initramfs images

The new building code included the rootfs twice when building tplink initramfs images.
To make it more readable move initramfs into an own build step
Build/mktplinkfw-initramfs.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

SVN-Revision: 45491