utils/spidev_test: build package directly from Linux

utils/spidev_test: build package directly from Linux

Jeff Kletsky noted in his patch titled:
"utils/spidev_test: Update to current source from upstream Linux"
that the spidev_test utility OpenWrt ships is severly out of date.

Instead of updating the spidev_test.c from the current kernel,
this patch replaces the package building code to utilize the
very file that gets shipped with the kernel we compiling for
anyway much like the "perf" package already does.

Reported-by: Jeff Kletsky <git-commits@allycomm.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>