build: Install python3 in korg images

If enabling BPF-related config options, tools/lib/bpf needs python3 to
be able to generate bpf_helper_defs.h. As such, include it in korg
images.

Author: rnav

build/korg/Dockerfile

@@ -27,6 +27,7 @@ RUN apt-get -q -y update && \
       libssl-dev \
       lzop \
       make \
+      python3 \
       openssl \
       u-boot-tools \
       rename \