efi_loader: disk: add efi_disk_is_system_part()

This function will check if a given handle to device is an EFI system
partition. It will be utilised in implementing capsule-on-disk feature.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Add function description. Return bool.
Reviewed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>