docs: Add documentation for define extra partition subtypes through build system

docs/en/api-guides/build-system.rst

@@ -343,6 +343,7 @@ from the component CMakeLists.txt:
 - ``IDF_VERSION_MAJOR``, ``IDF_VERSION_MINOR``, ``IDF_VERSION_PATCH``: Components of ESP-IDF version, to be used in conditional expressions. Note that this information is less precise than that provided by ``IDF_VER`` variable. ``v4.0-dev-*``, ``v4.0-beta1``, ``v4.0-rc1`` and ``v4.0`` will all have the same values of ``IDF_VERSION_*`` variables, but different ``IDF_VER`` values.
 - ``IDF_TARGET``: Name of the target for which the project is being built.
 - ``PROJECT_VER``: Project version.
+- ``EXTRA_PARTITION_SUBTYPES``: CMake list of extra partition subtypes. Each subtype description is a comma separated string with ``type_name, subtype_name, numeric_value`` format. Components may add new subtypes by appending them to this list.
 
   * If :ref:`CONFIG_APP_PROJECT_VER_FROM_CONFIG` option is set, the value of :ref:`CONFIG_APP_PROJECT_VER` will be used.
   * Else, if ``PROJECT_VER`` variable is set in project CMakeLists.txt file, its value will be used.

docs/en/api-guides/partition-tables.rst

@@ -131,6 +131,11 @@ See enum :cpp:type:`esp_partition_subtype_t` for the full list of subtypes defin
 
   Note that when writing in C++, an application-defined subtype value requires casting to type :cpp:type:`esp_partition_subtype_t` in order to use it with the :ref:`partition API<api-reference-partition-table>`.
 
+Extra Partition SubTypes
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+A component can define a new partition subtype by setting the ``EXTRA_PARTITION_SUBTYPES`` property. This property is a CMake list, each entry of which is a comma separated string with ``<type>, <subtype>, <value>`` format. The build system uses this property to add extra subtypes and creates fields named ``ESP_PARTITION_SUBTYPE_<type>_<subtype>`` in :cpp:type:`esp_partition_type_t`. The project can use this subtype to define partitions in the partitions table CSV file and use the new fields in :cpp:type:`esp_partition_type_t`.
+
 Offset & Size
 ~~~~~~~~~~~~~