Fix Arduino Serial monitor output

Thanks to @me-no-dev, the serial monitor issue was traced down to
DTR/RTS settings in boards.txt.  Clean those up and the monitor seems to
work perfectly now.

boards.txt

@@ -15,8 +15,8 @@ rpipico.upload.tool=uf2conv
 rpipico.upload.maximum_data_size=81920
 rpipico.upload.wait_for_upload_port=true
 rpipico.upload.erase_cmd=
-rpipico.serial.disableDTR=true
-rpipico.serial.disableRTS=true
+rpipico.serial.disableDTR=false
+rpipico.serial.disableRTS=false
 rpipico.build.led=
 rpipico.build.core=rp2040
 rpipico.build.mcu=rp2040