cc2538dk: Make it possible to override SLIP_ARCH_CONF_ENABLED

As the comment in contiki-conf.h says, the automatic definition of
SLIP_ARCH_CONF_ENABLED works only if UIP_FALLBACK_INTERFACE is tied to SLIP. If
UIP_FALLBACK_INTERFACE is set to another interface, SLIP_ARCH_CONF_ENABLED is
still automatically set to 1, leading to unwanted SLIP_END characters from
dbg.c:putchar() being printed on the UART.

This change makes it possible to force the definition of SLIP_ARCH_CONF_ENABLED
(e.g. from project-conf.h), so that it can be disabled if UIP_FALLBACK_INTERFACE
is used with something else than SLIP.