Contiki: undefine COAP_MAX_BLOCK_SZX before redefine
Contiki: undefine COAP_MAX_BLOCK_SZX before redefine

COAP_MAX_BLOCK_SZX is defined to "2" already before a define rule for
CONTIKI_TARGET_CC2538DK, just add a simply undefine line before the
redefine of COAP_MAX_BLOCK_SZX to make the compiler happy.

authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,gebart/libcoap,gebart/libcoap