LAVA health check wastes 30 seconds waiting for initramfs shell

Issue Background

LAVA's health check uses an initramfs built (partly) with initramfs-tools. The init script in this initramfs normally attempts to mount and pivot into a 'real' system, but LAVA doesn't use that. Instead it sets the kernel 'root' parameter to a nonexistent device ('root=/dev/ram0'), waits for the init script to print an error message, and then uses the recovery shell.

Issue description

The init script waits at least 30 seconds before giving up and opening a recovery shell, which is a waste of time. initramfs-tools supports a 'break' parameter which can be used to start a shell before the point where it attempts to mount any devices.

Acceptance criteria

• Health check starts a shell in initramfs without the wait
• Any other tests that start a shell in initramfs do the same
• No regression in health check or the other tests