x86/boot: Add back some padding for the CRC-32 checksum

Even though no uses of the bzImage CRC-32 checksum are known, ensure
that the last 4 bytes of the image are unused zero bytes, so that the
checksum can be generated post-build if needed.

[ mingo: Added the 'obsolete' qualifier to the comment. ]

Suggested-by: "H. Peter Anvin" <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ardbiesheuvel

arch/x86/boot/compressed/vmlinux.lds.S

@@ -48,7 +48,8 @@ SECTIONS
 		*(.data)
 		*(.data.*)
 
-		. = ALIGN(0x200);
+		/* Add 4 bytes of extra space for the obsolete CRC-32 checksum */
+		. = ALIGN(. + 4, 0x200);
 		_edata = . ;
 	}
 	. = ALIGN(L1_CACHE_BYTES);