[bzimage] Support old (pre-2.00 bootloader) Linux kernel formats

[bzimage] Support old (pre-2.00 bootloader) Linux kernel formats

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).