This allows gPXE to load memtest86, which is packaged as an old kernel.
Split all code that directly touches the kernel headers out into
bzimage_parse_header() and bzimage_update_header(), to reduce code
size and offset the cost of supporting older kernels.
Total cost of this feature: 11 bytes (uncompressed).
2.6.22+ kernels have an extra field in the bzimage_header structure to
indicate the maximum permitted command-line length. Use this if it is
available.
