packages: use SPDX short identifier for zlib license

packages: use SPDX short identifier for zlib license

We want to use SPDX identifier for license string as much as possible.
SPDX short identifier for zlib license is Zlib.

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

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