so-called external modules are being moved from the top level to a
sub-directory in external/

The script bin_library_info is collecting among other thing the
sha of the tree object that represent the top level of the module
itself... so we need to account for the fact that such module
is not necessarily at $SRCDIR/<module> but can be at
$SRCDIR/external/module

Change-Id: I6c6912a51819669bde512fa8acad7ac82eb50334