boot/efi: Optionally sign our EFI bootloader

* The private keys are in possession of Haiku, Inc.

Change-Id: I3b5b004e1dce0102f8a65f6d682f7e428845efe8
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4936
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>