So far the output of "sunxi-fel -h" was the only source of information
about sunxi-fel's command line parameters, and the description was
rather terse.
Add a manpage that describes the purpose of sunxi-fel and its options it
a bit more detail.
Amend the Makefile to install the manpage into the usual location.
Signed-off-by: Andre Przywara <osp@andrep.de>
