This commit introduces the first Thread 1.2 feature - enhanced keep
alive.
This commit also introduces the build config
OPENTHREAD_CONFIG_THREAD_VERSION to include/exclude Thread 1.2 code.
This commit adds new parameter `OT_CONFIG` to main `CMakeLists.txt`
file. This parameter allows users to specify a project-specific core
config header file (mapped to `OPENTHREAD_PROJECT_CORE_CONFIG_FILE`)
during build configuration.
The default value for this parameter is "none". When not specified
(value is "none"), a platform cmake file may choose to change
`OT_COFNIG` variable to provide its own core config header file. This
commit updates platform 'posix' and 'cc2548' cmake files to provide
platform specific header files when `OT_CONFIG` is not specified by
user.
This is important for allowing cmake to properly configure itself,
knowing what cflags will be passed. Also I removed all of the
public modifications of CMAKE_CXX_FLAGS, to better control where
the flags are actually applied.
The linker file is an exception to the toolchain file, as it
depends on symbols that are declared in the startup file
(startup-gcc.c for cc2538). Therefore it has been added as a
PUBLIC requirement for the cc2538 platform layer.
