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

sys/boot/efi/boot1/generate-fat.sh

@@ -44,6 +44,8 @@ mkdir -p stub/efi/boot
 
 # Make a dummy file for boot1
 echo 'Boot1 START' | dd of=stub/efi/boot/$FILENAME cbs=$BOOT1_SIZE count=1 conv=block
+# Provide a fallback startup.nsh
+echo $FILENAME > stub/efi/boot/startup.nsh
 
 umount stub
 mdconfig -d -u $DEVICE