add modbus slave support

add modbus slave support

components/connectivity/Modbus/3rdparty/freemodbus-v1.6/doc/tips.dox

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+/*! \page sec_tips Tips
+
+This page provides some tips for using the FreeModbus protocol stack.
+
+\section sec_tips_memory Reducing memory requirements 
+The memory requirements of FreeModbus can be tuned in the following way. These are basic tricks and can easily be done:
+
+ - Decided if you need RTU, ASCII and TCP at the same time. If not disable
+   them in the file mbconfig.h by settings the respective options
+   MB_RTU_ENABLED, MB_ASCII_ENABLED and MB_TCP_ENABLED to zero.
+ - If you don't need all Modbus functions disable them in the file mbconfig.h.
+   This will reduce code requirements. 
+ - Set the variable MB_FUNC_HANDLERS_MAX in mbconfig.h to the number
+   of functions codes you want to support.
+
+If you have stronger limits you can also try the following options. Note that this options have an impact on the features of the protocol stack.
+
+ - Use some compiler directive to put the mapping of function codes to
+   handler functions into the flash memory of you CPU. You can find this
+   table in the file mb.c at the top of the file. The static variable is
+   named xFuncHandlers.
+
+ - Reduce the size of the RTU buffer. In this case longer frames will 
+   result in an error (Your device will drop all these frames). This is
+   possible if you will never get read/write requests with that number
+   of registers or your total amount of registers is small anyway.
+
+ - You could also remove some function pointers which make the protocol
+   stack configurable and replace them by the functions itself. For
+   example if you only want to use RTU remove the callback functions from
+   the porting layer and fill in the appropriate calls. This will save
+   the space for all function pointers.
+*/