ArmPlatformPkg: Fix EDK2_DSC check in Scripts/Makefile
With GNU Make 4.2.1, ifeq ($(EDK2_DSC),"") doesn't catch the case where
EDK2_DSC isn't defined. So, switch to using ifndef.
Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>