doc: application: make default app a workspace app

It is well known that putting your application inside the west
workspace where zephyr is installed, i.e. defining a workspace
application, makes it easier to use west build, since you don't have
to juggle setting ZEPHYR_BASE appropriately.

Therefore, recommend doing thing this way, while leaving a hint about
how to do something else.

The current state of affairs where the application is assumed to be in
$HOME/app is longstanding and precedes the introduction of west to
zephyr, and I think it's overdue for the page to get with the times
and use conventions that work well with west: our reference
application has been a workspace application for almost 2 years at
this point.

Create a new meta-variable <app> to describe the location of the
application to keep things short and make it clearer that the actual
location on the file system doesn't matter as long as things are set
up properly.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>

doc/develop/application/index.rst

@@ -5,12 +5,18 @@ Application Development
 
 .. note::
 
-   In this document, we'll assume your **application directory** is
-   :file:`<home>/app`, and that its **build directory** is
-   :file:`<home>/app/build`.
-   (These terms are defined in the following Overview.)
-   On Linux/macOS, <home> is equivalent to ``~``, whereas on Windows it's
-   ``%userprofile%``.
+   In this document, we'll assume:
+
+   - your **application directory**, :file:`<app>`, is something like :file:`<home>/zephyrproject/app`
+   - its **build directory** is :file:`<app>/build`
+
+   These terms are defined below. On Linux/macOS, <home> is equivalent to
+   ``~``. On Windows, it's ``%userprofile%``.
+
+   Keeping your application inside the workspace (:file:`<home>/zephyrproject`)
+   makes it easier to use ``west build`` and other commands with it. (You can
+   put your application anywhere as long as :ref:`ZEPHYR_BASE
+   <important-build-vars>` is set appropriately, though.)
 
 Overview
 ********
@@ -33,7 +39,7 @@ An application in its simplest form has the following contents:
 
 .. code-block:: none
 
-   <home>/app
+   <app>
    ├── CMakeLists.txt
    ├── prj.conf
    └── src
@@ -645,14 +651,7 @@ Using CMake directly:
 Basics
 ======
 
-.. note::
-
-   In the below example, ``west`` is used outside of a west workspace. For this
-   to work, you must set the ``ZEPHYR_BASE`` environment variable to the path
-   of your zephyr git repository, using one of the methods on the
-   :ref:`Environment Variables <env_vars>` page.
-
-#. Navigate to the application directory :file:`<home>/app`.
+#. Navigate to the application directory :file:`<app>`.
 #. Enter the following commands to build the application's :file:`zephyr.elf`
    image for the board specified in the command-line parameters:
 
@@ -691,7 +690,7 @@ When using the Ninja generator a build directory looks like this:
 
 .. code-block:: none
 
-   <home>/app/build
+   <app>/build
    ├── build.ninja
    ├── CMakeCache.txt
    ├── CMakeFiles
@@ -751,7 +750,7 @@ the build system to rebuild the entire application from scratch with the
 following procedure:
 
 #. Open a terminal console on your host computer, and navigate to the
-   build directory :file:`<home>/app/build`.
+   build directory :file:`<app>/build`.
 
 #. Enter one of the following commands, depending on whether you want to use
    ``west`` or ``cmake`` directly to delete the application's generated
@@ -844,7 +843,7 @@ hardware:
    this via USB.
 
 #. Run one of these console commands from the build directory,
-   :file:`<home>/app/build`, to flash the compiled Zephyr image and run it on
+   :file:`<app>/build`, to flash the compiled Zephyr image and run it on
    your board:
 
    .. code-block:: console
@@ -894,7 +893,7 @@ hardware. Follow these instructions to run an application via QEMU:
    - ``qemu_cortex_m3`` to emulate running on an ARM Cortex M3-based board
 
 #. Run one of these console commands from the build directory,
-   :file:`<home>/app/build`, to run the Zephyr binary in QEMU:
+   :file:`<app>/build`, to run the Zephyr binary in QEMU:
 
    .. code-block:: console