rdinfra/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: I2038545c723bc48ab7084cd032a39d59ad1c9c22
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>