functions: fix unmounting for local builds

When building locally using the auto/build script unmounting fails.

Avoid mounting via bind. Mount mountpoint/dev as devtmpfs file system and
mountpoint/dev/pts as devpts file system.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

live-build/functions

@@ -131,7 +131,8 @@ setup_mountpoint() {
         chown root:kmem /dev/mem
     fi
 
-    mount --rbind /dev "$mountpoint/dev"
+    mount dev-live -t devtmpfs "$mountpoint/dev"
+    mount devpts-live -t devpts -o nodev,nosuid "$mountpoint/dev/pts"
     mount proc-live -t proc "$mountpoint/proc"
     mount sysfs-live -t sysfs "$mountpoint/sys"
     mount securityfs -t securityfs "$mountpoint/sys/kernel/security"