cli: Implement "fastpath sut configure" command

Implement "fastpath sut configure" (and "fastpath sut unconfigure")
commands. The former installs a specified kernel on the SUT, configures
the command line, sets up any sysctls and runs an optional bootscript
after reboot. The latter restores a system to its state before
configuring it.

When configuring, we assume a system that uses grub as the bootloader.
We go to great lengths to install the kernel in such a way that we can
recover back to the old kernel if it turns out to be buggy. Given we
only have an SSH connection, this can't work in 100% of situations but
it should work effectively in practice.

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