doc: Edit the application development common procedures.

Corrected the :envvar: markup. Rewrote the text to be less patronizing.

Change-Id: I09ec541ae41ad682106dc73a6e4495216dfa34d3
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>

doc/application/apps_common_procedures.rst

@@ -0,0 +1,49 @@
+.. _apps_common_procedures:
+
+Common Procedures
+#################
+
+Instructions that are common to many procedures are provided here
+and referred to from the procedures that require them.
+
+Procedures
+**********
+
+* `Setting Environment Variables`_
+
+Setting Environment Variables
+=============================
+
+Set environment variables every time you open a terminal console to work on
+|codename| applications.
+
+Steps
+-----
+
+1. In a shell terminal console, enter an :command:`export` command that is
+  consistent with your toolchain.
+
+   a) For a Yocto toolchain:
+
+    .. code-block:: bash
+
+     $ export ZEPHYR_GCC_VARIANT=yocto
+     $ export YOCTO_SDK_INSTALL_DIR=<yocto-installation-path>
+
+   Go to Step 2.
+
+   b) For an Xtools toolchain:
+
+    .. code-block:: bash
+
+     $ export ZEPHYR_GCC_VARIANT=xtools
+     $ export XTOOLS_TOOLCHAIN_PATH=/opt/otc_viper/x-tools/
+     $ export QEMU_BIN_PATH=/usr/local/bin
+
+2. Navigate to the |codename| installation directory and enter:
+
+  .. code-block:: bash
+
+   $ source zephyr-env.sh
+
+  To set the environment variable :envvar:`$(ZEPHYR_BASE)`.