imx-test: Fix build with the work-shared kernel source
The Kernel sources has now been moved out of sstate control and to
make this easier to spot this has been moved to tmp/work-shared/
directory.
One good consequence of this change is that the built artifacts are
controlled by the Linux build system itself and its path is now
exported in STAGING_KERNEL_BUILDDIR variable.
This path adapts the build params to this new layout.
Change-Id: Ie27ef289aa3ae4c2593303ccbc450e0fd53b49cd Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
