[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).

src/arch/i386/include/bzimage.h

@@ -75,6 +75,9 @@ struct bzimage_header {
 /** Offset of bzImage header within kernel image */
 #define BZI_HDR_OFFSET 0x1f1
 
+/** bzImage boot flag value */
+#define BZI_BOOT_FLAG 0xaa55
+
 /** bzImage magic signature value */
 #define BZI_SIGNATURE 0x53726448