vm2c: make sure we are not running an outdated initramfs
Some people or CI jobs may update vm2c but fail to upgrade their kernel/initramfs, which may lead to vm2c generating an invalid command line for boot2container.
I suggest we try detecting the version of boot2container in vm2c by adding the necessary code to perform the equivalent of bsdtar -Oxf out/initramfs.linux_amd64.cpio.xz etc/b2c.version
, and warn if the version is older than the the latest released version.
We may also use this feature to prevent running if we know we are not compatible anymore with this version.