docs: remove OE_INIT_FILE variable

There's probably no need for such a variable (the name of the script is
unlikely to change any time soon) and not all instances of
oe-init-build-env were actually using this variable.

For consistency sake, let's just remove the OE_INIT_FILE variable.

Suggested-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
(From yocto-docs rev: 6fd4421283005b0ecc980e9ef25770d383b93937)

Signed-off-by: Quentin Schulz <foss@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/bsp-guide/bsp.rst

@@ -233,7 +233,7 @@ section.
    setup script to define the OpenEmbedded build environment on your
    build host. ::
 
-      $ source &OE_INIT_FILE;
+      $ source oe-init-build-env
 
    Among other things, the script creates the :term:`Build Directory`, which is
    ``build`` in this case and is located in the :term:`Source Directory`.  After