Bring back the SECONDARY_STORAGE variable

This variable is a requirement in KK as newly introduced APIs
such as Context.getExternalFilesDirs uses it to retrieve
secondary storage locations on the device.

Change-Id: I17708c93498da5979c5742cad32232a34490bf5f
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_device_samsung_smdk4412-common.git into cm-11.0