[autoboot] Allow a SAN boot as a fallback if a filename boot returns

Currently, if both a filename and root-path are present, iPXE will
hook the SAN device but will only attempt to boot from the filename.
Change this behaviour so that both are attempted.  Users who want to
avoid booting from the SAN as a fallback can do so via the existing
"skip-san-boot" setting.

This allows for seamless deployment to a SAN target using Windows
Deployment Services (and similar products).  A user simply has to
define the root-path option in DHCP and then use WDS to deploy the
system.  No further configuration should be required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

 		root_path = NULL;
 	}
 
+	/* Check that we have something to boot */
+	if ( ! ( filename || root_path ) ) {
+		rc = -ENOENT_BOOT;
+		printf ( "Nothing to boot: %s\n", strerror ( rc ) );
+		goto err_no_boot;
+	}
+
 	/* Hook SAN device, if applicable */
 	if ( root_path ) {
 		drive = san_hook ( root_path, 0 );
 		goto err_san_describe;
 	}
 
-	/* Attempt filename or SAN boot as applicable */
+	/* Allow a root-path-only boot with skip-san enabled to succeed */
+	rc = 0;
+
+	/* Attempt filename boot if applicable */
 	if ( filename ) {
 		if ( ( rc = imgdownload ( image, filename,
 					  register_and_autoexec_image ) ) !=0){
 			printf ( "\nCould not chain image: %s\n",
 				 strerror ( rc ) );
+			/* Fall through to (possibly) attempt a SAN boot
+			 * as a fallback.  If no SAN boot is attempted,
+			 * our status will become the return status.
+			 */
 		} else {
 			/* Always print an extra newline, because we
 			 * don't know where the NBP may have left the
 			 */
 			printf ( "\n" );
 		}
-	} else if ( root_path ) {
+	}
+
+	/* Attempt SAN boot if applicable */
+	if ( root_path ) {
 		if ( fetch_intz_setting ( NULL, &skip_san_boot_setting) == 0 ) {
 			printf ( "Booting from SAN device %#02x\n", drive );
 			rc = san_boot ( drive );
 		} else {
 			printf ( "Skipping boot from SAN device %#02x\n",
 				 drive );
-			rc = 0;
+			/* Avoid overwriting a possible failure status
+			 * from a filename boot.
+			 */
 		}
-	} else {
-		rc = -ENOENT_BOOT;
-		printf ( "Nothing to boot: %s\n", strerror ( rc ) );
 	}
 
  err_san_describe:
 		}
 	}
  err_san_hook:
+ err_no_boot:
 	image_put ( image );
  err_alloc_image:
 	return rc;