boot, linux, package: use SPDX short identifier for GPLv2/GPLv2+

boot, linux, package: use SPDX short identifier for GPLv2/GPLv2+

We want to use SPDX identifier for license strings as much as possible.
SPDX short identifier for GPLv2/GPLv2+ is GPL-2.0/GPL-2.0+.

This change is done by using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/\<GPLv2\>/GPL-2.0/g'

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