sbsign: Add --detached option to create detached PKCS7 signatures

Add an option (--detached) to sbsign, which creates a detached
signature, rather than embedding it in the PE/COFF signature table.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>

image.c

@@ -362,3 +362,19 @@ out:
 	close(fd);
 	return !rc;
 }
+
+int image_write_detached(struct image *image, const char *filename)
+{
+	int fd, rc;
+
+	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
+	if (fd < 0) {
+		perror("open");
+		return -1;
+	}
+
+	rc = write_all(fd, image->sigbuf, image->sigsize);
+
+	close(fd);
+	return !rc;
+}