bl unified: use cable connected instead of check available

This supersedes #852 and makes it build for all targets

PIOS_USB_CheckAvailable() checks for both cable connected
as well as whether the device is fully enumerated. This
more stringent check takes a more variable amount of time
on different computers.

It is much more appropriate to simply check whether the
USB cable is connected to determine whether we should wait
further in the bootloader.

Also bumped the bootloader version on affected boards to 0x81.