lib: create empty lib subsystem

Create lib subsystem with empty Kconfig menu, subsystem build file.
Updated top-level build and documentation to include subsystem.
This was created to provide an example of how the `lib/` directory
can be connected to the Zephyr build system through `module.yml`,
and so that CI can verify that the extension settings used here
work with these sub-trees.
.
The approach follows the pattern of paralleling the Zephyr tree.
An empty subsystem was implemented rather than a non-empty one
in order to isolate the core changes from any specific library.
This provides a clean reference for users.
.
It was assumed that some users will want to strip out the `lib/`
subsystem separate from existing subsystems, and vice versa.
.
This was verified by:
  - visually verifying a clean build of:
      `west build -b custom_plank -p always example-application/app/`
  - visually verifying the subsystem appeared in the correct
    location in a clean build of:
      `west build -b custom_plank -p always example-application/app/`

Signed-off-by: Gregory Shue <gregory.shue@legrand.us>

CMakeLists.txt

@@ -5,3 +5,4 @@
 # as the module CMake entry point (see zephyr/module.yml).
 
 add_subdirectory(drivers)
+add_subdirectory(lib)

Kconfig

@@ -6,3 +6,4 @@
 # module options by going to Zephyr -> Modules in Kconfig.
 
 rsource "drivers/Kconfig"
+rsource "lib/Kconfig"

README.md

@@ -10,6 +10,7 @@ applications. Some of the features demonstrated in this example are:
 - [Custom boards][board_porting]
 - Custom [devicetree bindings][bindings]
 - Out-of-tree [drivers][drivers]
+- Out-of-tree libraries
 - Example CI configuration (using Github Actions)
 - Custom [west extension][west_ext]
 

lib/CMakeLists.txt

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# The visibility and compilation of libraries may be controlled
+# with a Kconfig setting as follows:
+#
+#   add_subdirectory_ifdef(CONFIG_CUSTOM_LIB              custom_lib)

lib/Kconfig

@@ -0,0 +1,6 @@
+# Copyright (c) 2021 Legrand North America, LLC.
+# SPDX-License-Identifier: Apache-2.0
+
+menu "Libraries"
+
+endmenu