boot1.efifat: provide a fallback startup.nsh

In case the firmware falls through to executing startup.sh, populate it
with the name of our boot loader. In normal operation this should not be
necessary but may allow the system to boot if it would otherwise just
remain at a shell prompt.

Reviewed by: andrew, imp, smh
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5878