boot, package: use SPDX short identifier for GPLv3/GPLv3+

boot, package: use SPDX short identifier for GPLv3/GPLv3+

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for GPLv3/GPLv3+ is GPL-3.0/GPL-3.0+.

This change is done using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/\<GPLv3\>/GPL-3.0/g'

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