license: cleanup: add SPDX Apache-2.0 license identifier

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>

cmake/compiler/host-gcc/generic.cmake

diff --git a/cmake/compiler/host-gcc/generic.cmake b/cmake/compiler/host-gcc/generic.cmake
index 0a44a1a..3f88956 100644
--- a/cmake/compiler/host-gcc/generic.cmake
+++ b/cmake/compiler/host-gcc/generic.cmake
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 # Configures CMake for using GCC
 
 find_program(CMAKE_C_COMPILER gcc)