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>