build: Fix application object files placement when building out of tree

For the application object files to get properly placed in the outdir
when the source files are outside of the zephyr tree we need to set
$srctree in kbuild to the parent dir of the application source code
rather than getting the default of $srctree being set to $ZEPHYR_BASE.

By doing this we are able to get the kbuild system to place the object
file results in the outdir rather than in the application source dir.

Jira: ZEP-369
Change-Id: I4d3ba67a4a38c15978d5bf7e1f0a912e9bf00f08
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>