Make EC2 AMIs use portsnap and freebsd-update mirrors hosted in AWS

This adjusts freebsd-update.conf and portsnap.conf files in EC2 AMIs to
point at the new AWS-hosted mirror network.

Approved by:	re (delphij)
MFC after:	1 month
Differential Revision:	https://reviews.freebsd.org/D25498

release/tools/ec2.conf

@@ -108,6 +108,12 @@ vm_extra_pre_umount() {
 		's/^#ChallengeResponseAuthentication yes/ChallengeResponseAuthentication no/' \
 		${DESTDIR}/etc/ssh/sshd_config
 
+	# Use FreeBSD Update and Portsnap mirrors hosted in AWS
+	sed -i '' -e 's/update.FreeBSD.org/aws.update.FreeBSD.org/' \
+		${DESTDIR}/etc/freebsd-update.conf
+	sed -i '' -e 's/portsnap.FreeBSD.org/aws.portsnap.FreeBSD.org/' \
+		${DESTDIR}/etc/portsnap.conf
+
 	# Use the NTP service provided by Amazon
 	sed -i '' -e 's/^pool/#pool/' \
 		-e '1,/^#server/s/^#server.*/server 169.254.169.123 iburst/' \