Generate the command option help text automatically from the list of
defined options.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

The online documentation (e.g. http://ipxe.org/cmd/ifopen), though not
yet complete, is far more comprehensive than could be provided within
the iPXE binary.  Save around 200 bytes (compressed) by removing the
command descriptions from the interactive help, and instead referring
users directly to the web page describing the relevant command.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Total saving: 261 bytes.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Originally-implemented-by: michael-dev@fami-braun.de
Signed-off-by: Michael Brown <mcb30@ipxe.org>