Fix incorrect test in sh. Bash implements the new test with [[ that
accepts the double equal, however, this is not a bash script. On Debian
and derivates, this will be run by dash.
Make the script portable by not using GNU extensions of xargs.
Call `validate-all-*.sh` scripts whenever possible.
Have bashate look for /usr/bin/env style shebang as well.
Remove useless validate-json.sh file.
Change-Id: Id2d697f2e9ddaa66e25f71f410200528c1e0cf6b
dc3687b18b