Restore /boot/efi to mtree.

Instead of whether /boot/efi exists, which it now always does, including
on systems that don't and can't use EFI, use whether /boot/efi is
present in fstab to signal to the installer that it is a valid ESP and
should be configured. This has essentially the same semantics, but allows
/boot/efi to be created unconditionally.

Reviewed by:	bdragon, imp
Tested by:	bdragon (ppc64)
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D29068

etc/mtree/BSD.root.dist

@@ -18,6 +18,8 @@
             rockchip              tags=package=runtime
             ..
         ..
+        efi
+        ..
         firmware
         ..
         loader.conf.d             tags=package=bootloader

usr.sbin/bsdinstall/scripts/bootconfig

@@ -47,7 +47,7 @@ if [ `uname -m` == powerpc ]; then
 fi
 
 # Update the ESP (EFI System Partition) with the new bootloader if we have an ESP
-if [ -d "$BSDINSTALL_CHROOT/boot/efi" ]; then
+if [ -n "$(awk '{if ($2=="/boot/efi") printf("%s\n",$1);}' $PATH_FSTAB)" ]; then
 	case $(uname -m) in
 	    arm64)	ARCHBOOTNAME=aa64 ;;
 	    amd64)	ARCHBOOTNAME=x64 ;;