cli: Extend configure module to reboot with test kernel

When the test kernel is installed, grub is configured so that it only
boots it once. Any additional reboot will cause the default kernel to be
booted instead. This behaviour is useful when the test kernel is buggy
and panics.

However, there are legitimate cases where we want to explicitly reboot
the system but still boot the test kernel. So let's add an API,
reboot_configured() to the configure module, which does exactly this.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>