package: use SPDX short identifier for license string in comments

package: use SPDX short identifier for license string in comments

We have started using SPDX short identifier for license string in
<PKG>_LICENSE variable. But license strings in comments are still
using old strings. For consistency, use SPDX short identifier in
comments as well.

Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>