boot, package: use SPDX short identifier for LGPLv2.1/LGPLv2.1+

boot, package: use SPDX short identifier for LGPLv2.1/LGPLv2.1+

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for LGPLv2.1/LGPLv2.1+ is LGPL-2.1/LGPL-2.1+.

This change is done using following command.
find . -name "*.mk" | xargs sed -ri '/LICENSE( )?[\+:]?=/s/LGPLv2.1(\+)?/LGPL-2.1\1/g'

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