Ability to exit and maybe enter USB storage without unplugging

[dreamlayers]

It would be nice to be able exit USB storage mode without unplugging the cable. That allows charging to continue, and it would also make testing a bit easier when developing.

The ability to enter USB storage is probably less useful for most people, and it's currently possible to do that by reloading Rockbox (eg. by selecting rockbox.ipod).

This exit feature existed from r20244 to r20911.  It was removed because with some operating systems it would allow unsafe removal. However, the cable can be unplugged at any moment, and Rockbox and the OS on the computer can do nothing to prevent it. Adding a button which actually does some checking for whether it's safe doesn't seem to make things an worse, as long as accidental activation is unlikely and nothing is encouraging users to disconnect at an unsafe time.

The USB HID mode that has been added since complicates this, but maybe some button combination or long press can be used. A combination or long press would also reduce the chance of accidental disconnects.

Some code is still left over: usb_storage_try_release_storage() in firmware/usbstack/usb_storage.c. It's currently not used for anything, but it seems usable for this feature.