Add raspi3 arm64 rootfs build support.

debian/changelog

@@ -6,6 +6,7 @@ livecd-rootfs (2.547) UNRELEASED; urgency=medium
     image/boot/uboot for raspi*.
   * Avoid issues of hard-linking to a symbolic vmlinuz as this can lead to a
     dangling symlink.
+  * Add raspi3 arm64 rootfs build support.
 
   [ Aleksandr Bogdanov ]
   * vagrant: disabling automatic console log file
@@ -13,7 +14,7 @@ livecd-rootfs (2.547) UNRELEASED; urgency=medium
   [ Michael Zanetti ]
   * Don't set QML2_IMPORT_PATH in environment for Ubuntu Touch images.
 
- -- Steve Langasek <steve.langasek@ubuntu.com>  Mon, 19 Nov 2018 10:39:33 -0800
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Fri, 23 Nov 2018 17:20:29 +0100
 
 livecd-rootfs (2.546) disco; urgency=medium
 

live-build/auto/config

@@ -786,7 +786,7 @@ case $PROJECT in
 esac
 
 case $ARCH in
-	armel|armhf)
+	armel|armhf|arm64)
 		KERNEL_FLAVOURS="${SUBARCH:-$KERNEL_FLAVOURS}"
 		case $SUBARCH in
 			dove)
@@ -896,7 +896,7 @@ echo "SUBPROJECT=\"${SUBPROJECT:-}\"" >> config/binary
 echo "LB_DISTRIBUTION=\"$SUITE\"" >> config/binary
 
 case $ARCH+$SUBARCH in
-	armhf+raspi2|armhf+raspi3)
+	armhf+raspi2|armhf+raspi3|arm64+raspi3)
 		cat > config/hooks/01-firmware-directory.chroot_early <<EOF
 #!/bin/sh -ex
 mkdir -p /boot/firmware