discover: Add petitboot,tty and track available consoles

Add the NVRAM parameter petitboot,tty which sets the default console to
use when booting a kernel.
In load_config() construct a list of available consoles depending on the
current platform. A future patch depending on firmware changes will
allow this list to be constructed dynamically.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>