layer: license: fix GPL-2.0 license identifiers changed upstream
Upstream commit
2456f523cf ("licenses: Update license file to match
current SPDX names") deprecated the usage of GPL-2.0 SPDX identifier,
removed GPL-2.0 license file and replaced it with GPL-2.0-only SPDX
identifier and text file.
Adjust layer recipes to use new SDPX identifier and text file.
imx-test recipe has license has been re-captured to use GPL-2.0-or-later
identifier, since it is required by the license text accompanying the
source code repository through "GPLv2 or later" statement in COPYING file.
Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>
