zephyr: Add files to support using with zephyr RTOS

zephyr/CMakeLists.txt

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+if (CONFIG_LIB_BOOST_SML)
+  zephyr_include_directories(../include)
+endif()

zephyr/Kconfig

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+config LIB_BOOST_SML
+    bool "Enable use of the boost::sml C++ state machine library"
+    default n
+    depends on CPP && !STD_CPP98 && !STD_CPP11 && !STD_CPP14
+    help
+      Include the boost::sml state machine language library,
+      see https://github.com/boost-ext/sml.

zephyr/README.md

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+# Support for using the boost::sml as a [Zephyr](https://docs.zephyrproject.org/latest/index.html) module
+
+To use add the reposiory path to your west maifest file, probably `west.yml`. See the zephyr documentation on [project manifests](https://docs.zephyrproject.org/latest/develop/west/manifest.html#projects) for details.
+
+
+``` yaml
+  projects:
+    - name: boost-sml
+      url: http://your.git.server/sml
+      revision: tag|branch|sha
+```
+
+Then run `west update` to pull in the code. You need to then add the following config options to `prj.conf` or whereever you manage your configuration.
+
+```
+CONFIG_CPP=y
+CONFIG_STD_CPP17=y
+CONFIG_REQUIRES_FULL_LIBCPP=y
+
+# Include boost sml state machine
+CONFIG_LIB_BOOST_SML=y
+```
+
+Then you can `#include <boost/sml.hpp>` to use the library.

zephyr/module.yaml

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+build:
+  cmake: zephyr
+  kconfig: zephyr/Kconfig