Modbus: allows modbus thread stack size customization with MODBUS_STACK_SIZE macro.

For example, add "-DMODBUS_STACK_SIZE=0x2000" to target CFLAGS to limit stack size to 128kB

input,in_topiccc,1,outer,0
input,in_topic1,1,DINT,1
output,out_topiccc,1,False,datatype0,0
output,out_topic1,1,False,DINT,1