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.
This variable is only used for the term recipe (and maybe for flashing). They
should not be evaluated if they are not needed and the user should not see a
warning that the port is not set if he does not use port (for example in make
all.)
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.
