cmake: Move SOC_* var defines to cmake/app/boilerplate.cmake

We need access to SOC_PATH in dts.cmake so we need to move the
definitions of SOC_NAME, SOC_SERIES, SOC_FAMILY, and SOC_PATH out of the
toplevel CMakeLists.txt and into cmake/app/boilerplate.cmake.  We place
them before we include cmake/dts.cmake so they will be available to use
in it.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>