ar71xx: use Image/BuildLoaderPatched for TPLINK-SAFELOADER images

Building the loader in Image/Build/TPLINK-SAFELOADER would break the image
builder as no target compiler is available in that stage.

Fixes #19580

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45855 3c298f89-4303-0410-b956-a3cf2f4a3e73