[RFC] disable suspend if booting from removable storage

suspend/resume is miserably broken (by design) when booting from removable storage (usb sticks). lets not allow xbmc to suspend/autosuspend

consoder the following scenario:
- user boots from usb stick
- user tries to suspend
- here usb host got reset. bye bye /dev/sda
- resume. /dev/sda is no more. bye bye /storage
- as a side effect - udevil would mount /dev/sda2 as /var/media/Whatever

with this commit, there will be NO "suspend" entry in xbmc shutdown menu. shutdown function CAN NOT be set to "suspend" and is forced as "shutdown"

needs testing on RPi, also need to ensure if it doesnt break something with netboot (this I can test myself next monday).

also, it has been reported several times that resume with NBD/ISCSI boot is broken. let me know if we need to disable suspend also for netboot.