config/powerpc-nvram: Don't require petitboot, prefix on all params

Currently, we only store nvram params with a "petitboot," prefix, so
that we don't grow the config list to an unbound size. However,
the "auto-boot?" param has no prefix.

Instead, remove the requirement for the "petitboot," prefix, and use an
array of known parameters instead.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>