package: use SPDX short identifier for GFDL family licenses

package: use SPDX short identifier for GFDL family licenses

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for GFDLv1.1/GFDLv1.2/GFDLv1.3 is GFDL-1.1/
GFDL-1.2/GFDL-1.3.

This change is done using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/GFDL(v)?([1]\.[1-3])(\+)?/GFDL-\2\3/g'

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>