support comedi whenever found

Remove the LINUX dependency from the gr-comedi CMake rules,
as, for example, comedi also has support on Debian kFreeBSD
which is not a LINUX platform.
find USB on all platforms

Keep the 'usb-1.0' as the preferred match, but also match simply
'usb' as provided on some platforms. Allows kFreeBSD system support.
Merge branch 'maint'
Merge branch 'master' into next