Rework ordering of cmake, so that libraries in subdirectories can add to internal lists as PICO_SDK_POST_LIST_FILES, PICO_CONFIG_HEADER_FILES etc. (#382)

Move generation of config header later, so it gets to include all values for PICO_CONFIG_HEADER_FILES

src/host/CMakeLists.txt

@@ -32,4 +32,6 @@ endmacro()
 set(PICO_HOST_DIR "${CMAKE_CURRENT_LIST_DIR}" CACHE INTERNAL "")
 function(pico_define_boot_stage2 NAME)
     add_executable(${NAME} ${PICO_HOST_DIR}/boot_stage2.c)
-endfunction()
+endfunction()
+
+pico_promote_common_scope_vars()