eth2538: Disable SLIP_ARCH_CONF_ENABLED

This avoids printing unwanted SLIP_END characters on the UART.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Update contiki

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>