build: use host LLVM toolchain

OpenWrt now employs eBPF components. These are built as we select all
non-shared packages.

Configure teh OpenWrt buildsystem to use the hots BPF toolchain and add
the required llvm compiler to the dependencies. This avoids increasing
the build-time as no eBPF toolchain needs to be built.

Signed-off-by: David Bauer <mail@david-bauer.net>