grub-efi, gummiboot: Emit correct path in startup.nsh

grub-efi, gummiboot: Emit correct path in startup.nsh

when we generate hddimage and use it to prepare SD card
using scripts/contrib/mkefidisk.sh as described here

http://elinux.org/Minnowboard:MinnowMaxYoctoProject

The content of startup.nsh contains

bootx64.efi

but this file is actually not in same dir as startup.nsh
its in /EFI/BOOT so for this to work entry in startup.nsh
should be

fs0:\EFI\BOOT\bootx64.efi

Otherwise the image is D-O-A

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>