doc: move README.bootmenu to HTML doc

Convert README.bootmenu to reStructured text and move it to
usage/bootmenu.rst.

Adjust the text concerning configuration settings as these now are managed
via Kconfig.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 063e970..d0f5a9f 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -11,4 +11,5 @@
 .. toctree::
    :maxdepth: 1
 
+   bootmenu
    pstore