boot, package: use SPDX short identifier for BSD-2c

boot, package: use SPDX short identifier for BSD-2c

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for BSD-2c is BSD-2-Clause.

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

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