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.
Due to a recent fix in shell.c, remote echo is now working as originally
intended. Local echo must be disabled or otherwise it will add up to the
remote one, causing a character-by-character double echoing.
TERMPROG and TERMFLAGS variables do not need to be exported as they are
used directly by a make receipe.
Exporting them prevents overwriting 'RIOT_TERMINAL' in the test.
