kbuild: re-order the code to not parse unnecessary variables

The top Makefile is divided into some sections such as mixed targets,
config targets, build targets, etc.

When we build mixed targets, Kbuild just invokes submake to process
them one by one.  In this case, compiler-related variables like CC,
KBUILD_CFLAGS, etc. are unneeded.

Check what kind of targets we are building first, and parse variables
for building only when necessary.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>