doc: move README.dfu to HTML documentation

Move README.dfu to doc/usage/dfu.rst and convert to reStructured text.

In the long run this page should be split into two. One for the overview
and one for the dfu command. UEFI capsule updates and dfutftp should be
integrated into the overview page.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>