Add stable commit to the setup FVP script

Instead of checking out latest on all Gits we have now identified a
couple of commits for all Gits (except optee_os) that could be
considered as stable commit.

Also added a script that cleans all Gits.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
You may want to use "read -t 10" instead of "sleep 10" so that you get the 10s timeout, the ability to interrupt with Ctrl+C, but also to speed up things by just pressing Return.