build: move from srctree to ZEPHYR_BASE for app include paths

$srctree for the application might not be set to be $ZEPHYR_BASE, use
$ZEPHYR_BASE instead to be more explicit in the build.

Change-Id: Iefa5ff59f246b584949329044f7a6531adc6ed62
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>