board/MynaPlayer odyssey: changed the way the file name and metadata of the images are constructed. ...

Currently, the envsubst tool substitutes the system date and a unique build ID generated with uuidgen.

This commit changes both the template for genimage and the post-image rootfs script to use git describe instead.

Also mentions the new naming scheme for the rauc bundles in the readme.

Closes #20.

Added the changelog in the readme.

README.md: Add instructions for building and installation