makefiles: clean up default-channel.inc.mk
makefiles: clean up default-channel.inc.mk

Clean up `default-channel.inc.mk` to only set the custom CFLAGS
if they are really needed.
This should help avoid cluttering `riotbuild.h`.

The default PAN ID configuration should be just as easy to set
as the default channel, so also add an option for that while we
are at it.

authmillenon/RIOT,yogo1212/RIOT,authmillenon/RIOT,kaspar030/RIOT,aeneby/RIOT,aeneby/RIOT,jasonatran/RIOT,smlng/RIOT,basilfx/RIOT,mtausig/RIOT,kYc0o/RIOT,kaspar030/RIOT,mtausig/RIOT,kYc0o/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,aeneby/RIOT,OTAkeys/RIOT,mtausig/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,kaspar030/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,smlng/RIOT,aeneby/RIOT,basilfx/RIOT,OlegHahm/RIOT,jasonatran/RIOT,yogo1212/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,josephnoir/RIOT,OTAkeys/RIOT,yogo1212/RIOT,authmillenon/RIOT,OlegHahm/RIOT,toonst/RIOT,basilfx/RIOT,toonst/RIOT,smlng/RIOT,kYc0o/RIOT,x3ro/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,x3ro/RIOT,ant9000/RIOT,basilfx/RIOT,OTAkeys/RIOT,x3ro/RIOT,yogo1212/RIOT,ant9000/RIOT,yogo1212/RIOT,toonst/RIOT,basilfx/RIOT,miri64/RIOT,OTAkeys/RIOT,ant9000/RIOT,mtausig/RIOT,RIOT-OS/RIOT,mtausig/RIOT,OlegHahm/RIOT,smlng/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,miri64/RIOT,miri64/RIOT,josephnoir/RIOT,x3ro/RIOT,aeneby/RIOT,OlegHahm/RIOT,authmillenon/RIOT,x3ro/RIOT,ant9000/RIOT,josephnoir/RIOT,toonst/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,miri64/RIOT,smlng/RIOT,toonst/RIOT