Add SPDX identifier to classes file

Signed-off-by: Stefano Babic <sbabic@denx.de>

classes/swupdate-common.bbclass

@@ -1,3 +1,7 @@
+# Copyright (C) 2015-2022 Stefano Babic
+#
+# SPDX-License-Identifier: GPLv3
+
 inherit swupdate-lib
 
 DEPENDS += "\

classes/swupdate-image.bbclass

@@ -1,5 +1,7 @@
 # Copyright (C) 2015-2021 Stefano Babic <sbabic@denx.de>
 #
+# SPDX-License-Identifier: GPLv3
+#
 # This class is thought to be used in an image recipe.
 # It generates a SWU file from the image itself
 # User *must* add a sw-descitpion file

classes/swupdate-lib.bbclass

@@ -1,3 +1,7 @@
+# Copyright (C) 2015-2022 Stefano Babic
+#
+# SPDX-License-Identifier: GPLv3
+
 DEPENDS += "python3-magic-native zstd-native"
 
 def swupdate_encrypt_file(f, out, key, ivt):

classes/swupdate.bbclass

@@ -1,5 +1,7 @@
 # Copyright (C) 2015-2021 Stefano Babic <sbabic@denx.de>
 #
+# SPDX-License-Identifier: GPLv3
+#
 # Some parts from the patch class
 #
 # swupdate allows to generate a compound image for the