bindesc: Add initial support for binary descriptor definition

Binary descriptors are data objects stored at a known location
of a binary image. They can be read by an external tool or image,
and are used mostly for build information: version, build time,
host information, etc.
This commit adds initial support for defining such descriptors.

Signed-off-by: Yonatan Schachter <yonatan.schachter@gmail.com>