Makefile.moddir_rules: Pass PJPROJECT_BUNDLED to download_externals

The download_externals script wasn't getting the PJPROJECT_BUNDLED
environment variable passed down to it so it wasn't downloading
the appropriate variant of res_digium_phone.  This could cause
crashes in the DPMA.

Change-Id: I5daa9369c7af1fd556d892e89a85f279a2533425