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.

board/MynaPlayer odyssey: removed initramfs. ...

This build no longer requires an initramfs. It has hence been removed entirely. The associated defconfig is deleted, as are the postbuild and postimage scripts and overlays. The patches have been relocated to the rootfs_patches directory. Also worth noting is that the machine-id setup is entirely gone from the build system.

Reworked certificates handling. The certs.sh.example contains a full example of this, and must be copied to certs.sh, and modified accordingly to reflect the real paths where the certificates are stored. A script that can be called by any board is creating symlinks from the real paths to corresponding statics targets for usage in genimage and postbuild scripts. The certs folder in the external tree must never be distributed in the repository, and is hence ignored via .gitignore.

board/MynaPlayer odyssey: create initial empty ext4 filesystem for storing rauc slot status, and add this to the full disk image.

Initial commit.