x86: Refactor the zboot innards so they can be reused with a vboot image

x86: Refactor the zboot innards so they can be reused with a vboot image

If vboot successfully verifies a kernel, it will leave it in place and
basically ready to boot. The zeropage table which is part of the x86 boot
protocol is at the end of the kernel, though, instead of the beginning, and
because the image is already in place there's no need to copy it around.
This change refactors the code which implements the zboot command so that
the configuration of the zeropage table and loading the pieces of the
kernel into memory are done separately. Also, because the command line goes
before the zeropage table in vboot which is somewhat incompatible with the
normal protocol, where to put the command line is a now a parameter instead
of being hard coded.

Signed-off-by: Gabe Black <gabeblack@chromium.org>