boot1: avoid using NULL device path

As of r323063 boot1 printed out the path & device from which it was
loaded, but uboot's EFI implementation lacked some support, resulting in
a NULL pointer and a crash. Check for a NULL pointer and avoid
reporting (and storing in the environment) the device and path in this
case.

Submitted by: Zakary Nafziger <worldofzak@gmail.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13038