binman: Add support for passing arguments to entries

binman: Add support for passing arguments to entries

Sometimes it is useful to pass binman the value of an entry property from
the command line. For example some entries need access to files and it is
not always convenient to put these filenames in the image definition
(device tree).

Add a -a option which can be used like this:

   -a<prop>=<value>

where

   <prop> is the property to set
   <value> is the value to set it to

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