grub-install: Check for arm-efi as a default target

grub-install: Check for arm-efi as a default target

Much like on x86, we can work out if the system is running on top of EFI
firmware. If so, return "arm-efi". If not, fall back to "arm-uboot" as
previously.

Split out the code to (maybe) load the efivar module and check for
/sys/firmware/efi into a common helper routine is_efi_system().

Signed-off-by: Steve McIntyre <93sam@debian.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>