Add /etc/autofs/special_efs to EC2 AMIs

Since Amazon Elastic File System is only available within AWS, it seems
more appropriate to have this added only in EC2 AMIs rather than
"polluting" non-EC2 images with it.

Reviewed by:	gjb
MFC after:	7 days
Relnotes:	Amazon EFS filesystems can be automounted by enabling autofs
		and placing "/efs -efs" into /etc/auto_master.
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D24791

release/tools/ec2.conf

@@ -113,6 +113,23 @@ vm_extra_pre_umount() {
 		-e '1,/^#server/s/^#server.*/server 169.254.169.123 iburst/' \
 		${DESTDIR}/etc/ntp.conf
 
+	# Provide a map for accessing Elastic File System mounts
+	cat > ${DESTDIR}/etc/autofs/special_efs <<'EOF'
+#!/bin/sh
+
+if [ $# -eq 0 ]; then
+        # No way to know which EFS filesystems exist and are
+        # accessible to this EC2 instance.
+        exit 0
+fi
+
+# Provide instructions on how to mount the requested filesystem.
+FS=$1
+REGION=`fetch -qo- http://169.254.169.254/latest/meta-data/placement/availability-zone | sed -e 's/[a-z]$//'`
+echo "-nfsv4,minorversion=1,oneopenown ${FS}.efs.${REGION}.amazonaws.com:/"
+EOF
+	chmod 755 ${DESTDIR}/etc/autofs/special_efs
+
 	# The first time the AMI boots, the installed "first boot" scripts
 	# should be allowed to run:
 	# * ec2_configinit (download and process EC2 user-data)