serial.inc.mk: disable local echo in picocom.

[jcarrano]

Contribution description

Due to a recent fix in shell.c (#10630), 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.

Testing procedure

Note: exit picocom by typing Ctrl-a Ctrl-x.

Run RIOT_TERMINAL=picocom BOARD=samr21-xpro make -C tests/shell term

Without this fix:

Type [C-a] [C-h] to see available commands
Terminal ready


> hheellpp

Command              Description
---------------------------------------
start_test           starts a test
end_test             ends a test
echo                 prints the input command
reboot               Reboot the node
ps                   Prints information about running threads.
> <type enter here>
<empty line that shoul not exist>
>

With this fix:

> help
Command              Description
---------------------------------------
start_test           starts a test
end_test             ends a test
echo                 prints the input command
reboot               Reboot the node
ps                   Prints information about running threads.
>  <note no empty line after typing enter here>
>

Issues/PRs references

See #10630, #10952

[kaspar030]

ACK.