arm-bsp/n1sdp: add support for startup.nsh file

UEFI EDK2 on N1SDP is unable to detect FS2 during boot resulting
in launching of EDK2 shell instead of launching grub.
The startup.nsh will force launching of grub.

Change-Id: I90426f2883251b7cc5fd4aa453ccfd606640d3bd
Signed-off-by: Khasim Syed Mohammed <khasim.mohammed@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>