[cc2538] add 1K stack space (#3385)
[cc2538] add 1K stack space (#3385)

In the observed deepest CoAP process when handling JOIN_FIN, the large
local variables in the nested calls may take up to 2.5K, among with
the three local CoAP headers would add about extra 1K since #3210 which
increases CoAP header from 128 to 512.

This commit adjusts stack to be of 5K by increasing 1K to help commissioning
process goes correctly.

openthread/openthread,erja-gp/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,jwhui/openthread,chshu/openthread,erja-gp/openthread,bukepo/openthread,openthread/openthread,chshu/openthread,erja-gp/openthread,chshu/openthread,abtink/openthread,srickardti/openthread,srickardti/openthread,librasungirl/openthread,librasungirl/openthread,turon/openthread,librasungirl/openthread,turon/openthread,lanyuwen/openthread,erja-gp/openthread,srickardti/openthread,lanyuwen/openthread,turon/openthread,erja-gp/openthread,lanyuwen/openthread,abtink/openthread,bukepo/openthread,chshu/openthread,bukepo/openthread,openthread/openthread,lanyuwen/openthread,bukepo/openthread,chshu/openthread,lanyuwen/openthread,abtink/openthread,lanyuwen/openthread,openthread/openthread,erja-gp/openthread,jwhui/openthread,jwhui/openthread,turon/openthread,srickardti/openthread