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>
Right, left-over from my debugging yesterday, thanks for catching this. I'll update and use Jerome's suggestion instead.