binman: Add support for bintools

binman: Add support for bintools

Binman requires various tools to actually work, such as 'lz4' to compress
data and 'futility' to sign Chrome OS firmware. At present these are
handled in an ad-hoc manner and there is no easy way to find out what
tools are needd to build an image, nor where to obtain them.

Add an implementation of 'bintool', a base class which implements this
functionality. When a bintool is required, it can be requested from this
module, then executed. When the tool is missing, it can provide a way to
obtain it.

Note that this uses Command directly, not the tools.Run() function. This
allows proper handling of missing tools and avoids needing to catch and
re-raise exceptions.

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