Ensure $TMP_BUILD_DIR is actually created.

Previously if mktemp failed in function mk_build_dir, die was not
called.  The check for the exit code was actually checking that the
previous export statement completed successfully, not the call to
mktemp.

Change-Id: I477a2ce75c87c8167883ce7aa342e93c40770e29

lib/common-functions

@@ -24,8 +24,9 @@ function tmpfs_check() {
 }
 
 function mk_build_dir () {
-  export TMP_BUILD_DIR=$(mktemp -t -d --tmpdir=${TMP_DIR:-/tmp} image.XXXXXXXX)
+  TMP_BUILD_DIR=$(mktemp -t -d --tmpdir=${TMP_DIR:-/tmp} image.XXXXXXXX)
   [ $? -eq 0 ] || die "Failed to create tmp directory"
+  export TMP_BUILD_DIR
   if tmpfs_check ; then
     sudo mount -t tmpfs tmpfs $TMP_BUILD_DIR
   fi