build: Split out building of application code

Seperate building of the application code from the zephyr OS code.  This
is in prep for both having a libzephyr.a for the OS and to fix where
the application object files get put when building out of tree.

Change-Id: I43f3b54d11f41aaf35350de7462c5cd556804092
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>