sgi/uefi: add a script for booting upto efi shell

There should be a provision to boot upto the efi shell but not beyond.
This is particularly useful for quick santiy checking of changes in
firmware. Add a new test script to boot upto the efi shell.

Change-Id: I193c081c88062d23c6aab29a8a896c3aeba851fa
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>