Usart channel selection for serial line debugging

Serial debug channel is selectable now via usart_choice, default is channel 0.

The first commit only removes trailing spaces from Configure.help
The help has been upted. As Syslog is an option with Debug, the explanatory text (serial-line) was dropped.

DEBUG_USE_USART is now a default value delivered by defconfig, so the hard setting in debug.c was removed.

It remains to be discussed if usart_count_used is required in line 90 and 109 of config.in

After this commit is accepted, antares, beteigeuze profiles and corresponding m4 files must be updated manually.