makefiles/serial.inc.mk: add PYTERMFLAGS variable
makefiles/serial.inc.mk: add PYTERMFLAGS variable

Add a variable for `pyterm` specific flags that are not handled by other
terminals.
This will prevent issues with boards that have options only supported by
`pyterm` and setting `pyterm` options from the environment.

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