The shim review board (which is the secure boot base loader) recommends using
ephemeral keys when signing the Linux Kernel. This commit enables the Kernel
build system to generate a one-time ephemeral key that is used to:
* sign all build-in Kernel modules
* sign all other out-of-tree Kernel modules
The key lives in /tmp and is destroyed after the build container exits and is
named: "VyOS build time autogenerated kernel key".
In addition the Kernel now uses CONFIG_MODULE_SIG_FORCE. This now makes it
unable to load any Kernel Module to the image that is NOT signed by the
ephemeral key.
