Add stable commits 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 commits.

Also added a script that cleans all Gits.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (FVP)