efi_loader: initialise partition_signature memory

Zero partition_signature in the efi_device_path_hard_drive_path
structure when signature_type is 0 (no signature) as required by the
UEFI specification.

This is required so that efi_dp_match() will work as expected
when doing memcmp() comparisons.  Previously uninitialised memory
would cause it not match nodes when it should have when the signature
type was not GUID.

Corrects a problem where the loaded image protocol would not return a
device path with MEDIA_DEVICE causing the OpenBSD bootloader to fail
on rpi_3 and other targets.

v2: Also handle signature_type 1 (MBR) as described in the specification

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Tested-by: Artturi Alm <artturi.alm@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>