Enable mounting of connected USB storage devices:

Thanks to CyanogenMod:
https://github.com/CyanogenMod/android_device_asus_deb/commit/0c741de590db991b43903a2e2e18fed5c85e9f71
https://github.com/CyanogenMod/android_device_asus_deb/commit/37ae93f8cb6163ec91478b7057c15b9e79c5512c