usb: gadget: mass_storage: added sysfs entry for cdrom to LUNs

This patch allows (rooted) applications to switch between USB-drive emulation and CDrom-emulation.
It is needed, because at the moment the HTC One has its CDrom emulation hardcoded (there is no way for applications to turn this off). That means the Mass Storage functionality of the device cannot run as a normal USB disk, but only as a CDrom device.

More specifically, it allows DriveDroid (https://play.google.com/store/apps/details?id=com.softwarebakery.drivedroid) to be properly used on the device.