Activate license_check.sh in Makefile, CI

Add license_check: to Makefile, and place into do_release:, test:, and do_rpi2_release: steps. Slight modification to support/license_check.sh, to check the state of $install_dir/LICENSE since the license_check: step in the Makefile is not directly dependant on the build: step due to the omnibus call rebuilding entirely at every call.

This is the final step of 3 in answering #1518 (closed) , via shell script. Closes #1518 (closed)