Add support for ESP-IDF platform complete with documentation

src/esp-idf/CMakeLists.txt

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+# The following five lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+set(srcs
+    "src/modbus-data.c"
+    "src/modbus-rtu.c"
+    "src/modbus-tcp.c"
+    "src/modbus.c")
+
+set(include_dirs src)
+
+set(priv_include_dirs ../)
+
+list(APPEND priv_include_dirs)
+
+add_prefix(srcs "${CMAKE_CURRENT_LIST_DIR}/libmodbus/" ${srcs})
+add_prefix(include_dirs "${CMAKE_CURRENT_LIST_DIR}/libmodbus/" ${include_dirs})
+add_prefix(priv_include_dirs "${CMAKE_CURRENT_LIST_DIR}/libmodbus/" ${priv_include_dirs})
+
+message(STATUS "DEBUG: Using libmodbus component folder: ${CMAKE_CURRENT_LIST_DIR}.")
+
+idf_component_register(SRCS "${srcs}"
+                    INCLUDE_DIRS "${include_dirs}"
+                    PRIV_INCLUDE_DIRS "${priv_include_dirs}"
+                    PRIV_REQUIRES driver vfs)
+

src/esp-idf/README.md

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+# Instructions to use with Espressif IoT Development Framework (ESP-IDF)
+
+## Adding libmodbus as a component
+
+- Create a subdirectory at the top level of your ESP-IDF project where you will
+place this and create a component, ie. `libmodbus`. This directory will be
+referred as *component directory* further down in this text.
+
+- Download the latest version of libmodbus with the method of your choice and
+unpack it under component directory in a subdirectory `libmodbus`
+
+- Copy the files supplied in this documentation directory to the component directory,
+namely:
+  - `CMakeLists.txt`: the CMake script that enumerates files and directories used
+     in the build as well as defines needed dependencies
+  - `component.mk`: the component build definition
+  - `config.h`: the library configuration, especially tailored for ESP-IDF. This is 
+    usually generated with the autoconf tool, but this is not present in ESP-IDF and
+    is therefore manually prepared and customized
+  - `idf_component.yml`: the component description file
+
+- Add a reference from your main project in the project top level `CMakeLists.txt` to
+  the newly added module with something like: `set(EXTRA_COMPONENT_DIRS libmodbus/)`. 
+  If you already have other components you may just add the reference to the newly
+  added component.
+
+- As the ESP-IDF does not provide a `nanosleep` function in its SDK, you should add
+  this in your project so you will be able to find it at linking time, for example:
+
+```
+int nanosleep(const struct timespec *req, struct timespec *_Nullable rem) {
+    return usleep(req->tv_sec*1000 + req->tv_nsec / 1000);
+}
+```
+
+Now you are almost ready to use libmodbus in your project!
+
+If you desire to use the library for serial communication, you will need to do a few
+more hardware configuration steps before using the `modbus_new_rtu` call, namely:
+
+- Configure, if needed, any pins for the used uart via `uart_set_pin`
+
+- Install the uart driver via the `uart_driver_install`
+
+- Configure, if needed, the uart mode (ie. set it to half duplex) via `uart_set_mode`
+
+These configurations are not included in libmodbus as they are highly hardware specific
+and would require a heavy change in the library interface.
+
+## Other details using libmodbus with ESP-IDF
+
+- The serial driver is implemented using the `vfs` virtual filesystem component. This
+  makes the changes needed for the library minimal, but may not be the most performant
+  solution.
+
+- The serial name (first parameter to `modbus_new_rtu`) should be a string containing
+  only the serial index (ie. `"1"` or `"2"`).
+
+- When using the TCP version be aware of the maximum number of sockets that can be
+  open on the platform: this is by default 10 and can be possibly raised to 16 in
+  a standard configuration. Please check the `LWIP_MAX_SOCKETS` configuration
+  variable.
+

src/esp-idf/component.mk

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+INCLUDEDIRS := src
+PRIV_INCLUDEDIRS := ../
+SRCDIRS := src
+
+COMPONENT_PRIV_INCLUDEDIRS = $(addprefix libmodbus/, \
+	$(PRIV_INCLUDEDIRS) \
+	)
+
+COMPONENT_SRCDIRS = $(addprefix libmodbus/, \
+	$(SRCDIRS) \
+	)
+
+COMPONENT_ADD_INCLUDEDIRS = $(addprefix libmodbus/, \
+	$(INCLUDEDIRS) \
+	)
+
+
+