environment: return external dir if secondary is not defined

If the device doesn't define a secondary storage, then return external storage
as secondary storage

Patchset 2: do not break the api

Change-Id: I9d94a01881f47977ef1a26c3648b3da1d8c63b57
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
Merge remote-tracking branch 'cyanogenmod/cm-12.0' into cm-12.0