bsdinstall: use gpt/efiboot0 label in /etc/fstab

Otherwise, boot will hang if the numbering of disks has changed since
initial install.

MFC after:	2 weeks
Sponsored by:	Axcient
Reviewed by:	brd
Differential Revision: https://reviews.freebsd.org/D35309

usr.sbin/bsdinstall/scripts/zfsboot

@@ -868,7 +868,7 @@ zfs_create_diskpart()
 
 			# We'll configure the ESP in bootconfig
 			if [ -z "$efibootpart" ]; then
-				efibootpart="/dev/$(echo $justaddedpart | cut -f 1 -d ' ')"
+				efibootpart="/dev/gpt/efiboot$index"
 				f_dprintf "$funcname: configuring ESP at [%s]" \
 				          "${efibootpart}"