x86: Fix support for booting bzImage

Add support for newer (up to 2.6.33) kernels

Add zboot command which takes the address of a bzImage as its first
argument and (optionally) the size of the bzImage as the second argument
(the second argument is needed for older kernels which do not include
the bzImage size in the header)

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>