[efi] Release SNP devices before starting SAN boot image

Release SNP devices to allow the SAN booted image to use our
EFI_SIMPLE_NETWORK_PROTOCOL instance, and to ensure that the image is
started at TPL_APPLICATION.

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

src/interface/efi/efi_block.c

 		goto err_sandev_find;
 	}
 
+	/* Release SNP devices */
+	efi_snp_release();
+
 	/* Connect all possible protocols */
 	efi_block_connect ( sandev );
 
 
 	bs->FreePool ( handles );
  err_locate_file_systems:
+	efi_snp_claim();
  err_sandev_find:
 	return rc;
 }