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

[TOPOLOGY]
NetName=None
Nodes=0x01
Node64Present=0x01
Node64Name=micromod
Node64DCFName=PEAK MicroMod.eds
EDSBaseName=eds