s390/boot: add secure boot trailer

This patch enhances the kernel image adding a trailer as required for
secure boot by future firmware versions.

Cc: <stable@vger.kernel.org> # 5.2+
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/boot/vmlinux.lds.S

@@ -102,8 +102,17 @@ SECTIONS
 		_compressed_start = .;
 		*(.vmlinux.bin.compressed)
 		_compressed_end = .;
-		FILL(0xff);
-		. = ALIGN(4096);
+	}
+
+#define SB_TRAILER_SIZE 32
+	/* Trailer needed for Secure Boot */
+	. += SB_TRAILER_SIZE; /* make sure .sb.trailer does not overwrite the previous section */
+	. = ALIGN(4096) - SB_TRAILER_SIZE;
+	.sb.trailer : {
+		QUAD(0)
+		QUAD(0)
+		QUAD(0)
+		QUAD(0x000000207a49504c)
 	}
 	_end = .;