Only create a tmpfs big enough for DIB_MIN_TMPFS

This will help to ensure images that would cause bug #1289582 would
still fail even on systems that have more RAM. Also this will ensure we
only reserve enough tmpfs space for this build.

Change-Id: Icece64e691d126a5fc89262897630fdbef1ffdf1

lib/common-functions

@@ -31,7 +31,7 @@ function mk_build_dir () {
   [ $? -eq 0 ] || die "Failed to create tmp directory"
   export TMP_BUILD_DIR
   if tmpfs_check ; then
-    sudo mount -t tmpfs tmpfs $TMP_BUILD_DIR
+    sudo mount -t tmpfs tmpfs -o size=$(($DIB_MIN_TMPFS*2))G $TMP_BUILD_DIR
   fi
   sudo chown $(id -u):$(id -g) $TMP_BUILD_DIR
   trap cleanup EXIT