bl unified: use cable connected instead of check available

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.
F4 USB: Add PIOS_USB_CableConnected method

This is used by the F1/F3 BL but not F4.  Note that it
behaves slightly differently than the other targets which
look for a USB SOF event.
BL: updates per review

Remove comment ask requested, although it seemed ok. Code is self
documenting anyway.
Merge pull request #983 from peabody124/bl_check_cable

bl unified: use cable connected instead of check available