CI: Allow building with internal toolchain

CI: Allow building with internal toolchain

This adds an option to build with internal toolchain. This can be used
to build targets which are currently not build by the OpenWrt build bots
and which needs their own toolchain build for every build.

Building the toolchain takes about 30 minutes compared to using the
external toolchain which takes some seconds.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 08f5283392674fe874c7f441128319263ce0d171)