doc/services/llext: Mention the LLEXT EDK

Mention the EDK existence, the build target and discuss some features.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>

doc/services/llext/index.rst

@@ -18,6 +18,66 @@ needed by the :c:func:`llext_load` function. An implementation over a buffer
 containing an ELF in addressable memory in memory is available as
 :c:struct:`llext_buf_loader`.
 
+LLEXT Extension Development Kit
+*******************************
+
+To build the extensions to be loaded by the llext subsystem, it's necessary to
+have access to the headers and compiler flags used by the main Zephyr
+application. While building an extension as part of the Zephyr application
+is straightforward, it can be more complex as a standalone project.
+
+The LLEXT Extension Development Kit (EDK) provides a set of headers and compile
+flags that can be used to build extensions as standalone projects. The EDK
+can be generated by the Zephyr build system by running the following command
+which uses the ``llext-edk`` target to generate the EDK:
+
+.. code-block:: shell
+
+    west build -t llext-edk
+
+The generated EDK can be found in the build directory under the ``zephyr``
+directory. It's a tarball that contains the headers and compile flags needed
+to build extensions. The extension developer can then include the headers
+and use the compile flags in their build system to build the extension.
+
+Compile flags
+-------------
+
+Convenience files ``cmake.cflags`` (for CMake based projects) and
+``Makefile.cflags`` are provided by the EDK containing the needed compile flags.
+The main flag is ``LLEXT_CFLAGS`` which contains the flags needed to build an
+extension. Also provided is a granular set of flags that can be used in support
+of different use cases, such as when building mocks for unit tests. The provided
+flags are:
+
+- ``LLEXT_CFLAGS``: Flags needed to build an extension.
+- ``LLEXT_ALL_INCLUDE_CFLAGS``: Include flags needed to include all headers.
+- ``LLEXT_INCLUDE_CFLAGS``: Include flags for non build generated headers.
+- ``LLEXT_GENERATED_INCLUDE_CFLAGS``: Include flags for build generated headers.
+- ``LLEXT_BASE_CFLAGS``: All other flags needed to build an extension,
+  excluding the include flags.
+- ``LLEXT_GENERATED_IMACROS_CFLAGS``: Include only ``-imacros`` flags for build
+  generated headers.
+
+LLEXT EDK Kconfig options
+-------------------------
+
+The LLEXT EDK can be configured using the following Kconfig options:
+
+:kconfig:option:`CONFIG_LLEXT_EDK_NAME`
+    The name of the generated EDK tarball.
+
+:kconfig:option:`CONFIG_LLEXT_EDK_USERSPACE_ONLY`
+    If set, the EDK will include headers that do not contain code to route
+    syscalls to the kernel. This is useful when building extensions that will
+    run exclusively in user mode.
+
+EDK Sample
+----------
+
+Refer to :zephyr:code-sample:`llext-edk` for an example of how to use the
+LLEXT EDK.
+
 API Reference
 *************