Update the files which contain no license information with the
'Apache-2.0' SPDX license identifier. Many source files in the tree are
missing licensing information, which makes it harder for compliance
tools to determine the correct license.
By default all files without license information are under the default
license of Zephyr, which is Apache version 2.
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Both variables were used (with the same value) interchangeably
throughout CMake files and per the discussion in GH issue,
ZEPHYR_BASE is preferred.
Also add a comment with explanation of one vs. the other.
Tested by building hello_world for several boards ensuring no errors.
Fixes#7173.
Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
