binman: Add gzip bintool

Add gzip bintool to binman to support on-the-fly compression of Linux
kernel images and FPGA bitstreams. The SPL basic fitImage implementation
supports only gzip decompression.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Rename the module and support this, since gzip.py is a system module:
Signed-off-by: Simon Glass <sjg@chromium.org>