Add cleanup function to handle unexpected build-linux.sh fail

In case of an unexpected exit(SIGINT, SIGHUP, etc), the linux
repository might be left in an unstable state where the branches
i.e. n1sdp-branch and ubuntu-branch might point to incorrect
commits. This will result in a build failure even after running
"clean" command.

This patch adds a trap to catch such situtaions and clean the linux
folder git branches before exiting.

Signed-off-by: Sahil Kaushal <Sahil.Kaushal@arm.com>