command: Let builtin command take precedence ...

In theory we can overwrite a builtin command with a script. However,
I don't know a single case where this has been done. Scripts are
often more unflexible than commands so it's unlikely that a script
can extend the functionality of a builtin command. Moreover, the
internal command is no longer accessible once it's overwritten by
a script.

Invert this logic so that a builtin command can overwrite an existing
script. This will help when the 'boot' script is converted to a
builting command. Then with old environments the builtin command will
be used instead of the script.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Introduce binfmt support ...

This will allow to execute any file and detect it's type to handle it.
This will allow to use shell for bootp bootfile or dfu.

You can register multiple hook for the same filetype. They will be execute
in the invert order of register. If a hook does not handle the file you just
return -ERESTARTNOHAND;

This is only available with hush parser.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>