cmake: Allow selecting toolchain file based on config

Refactor IDF "project" functionality under a wrapping of the default
"project" command, so we can tweak it a bit...

Will need more testing in other environments.

examples/get-started/hello_world/CMakeLists.txt

@@ -1,10 +1,8 @@
 # The following five lines of boilerplate have to be in your project's CMakeLists
 # in this exact order for cmake to work correctly
 cmake_minimum_required(VERSION 3.5)
-set(CMAKE_TOOLCHAIN_FILE $ENV{IDF_PATH}/toolchain.cmake)
-
-project(hello-world ASM C CXX)
 
 set(MAIN_SRCS main/hello_world_main.c)
 
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+project(hello-world)