classes: Add SPDX license identifiers

As stated in our top level license files, the license is MIT unless
otherwise stated. Add SPDX identifers accordingly. Replace older
license statementa with the standardised syntax. Also drop "All
Rights Reserved" expression as it isn't used now, doesn't mean anything
and is confusing.

(From OE-Core rev: 081a391fe09a21265881e39a2a496e4e10b4f80b)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/devtool-source.bbclass

@@ -1,6 +1,8 @@
 #
 # Copyright OpenEmbedded Contributors
 #
+# SPDX-License-Identifier: MIT
+#
 
 # Development tool - source extraction helper class
 #