binman: Add BintoolPacker class to bintool

Add a bintools base class for packers which compression / decompression
entry contents.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Dropped dead/untested code in version():
Signed-off-by: Simon Glass <sjg@chromium.org>