bootflow: Allow setting a cmdline arg with no value

This is supposed to be supported by the 'bootflow cmd' command, at
least according to the help. There is a 'bootflow cmd clear' but it is
often more intuitive to use 'bootcmd cmd set' with an empty value.

Update the command to pass BOOTFLOWCL_EMPTY in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>