spidev_test: new package

[Thomas: instead of building spidev_test directly from the source in
$(BR2_DL_DIR), use SPIDEV_TEST_EXTRACT_CMDS to copy the downloaded
source to the build directory, and build it from here. Allows to
manually tweak the source in the build directory for testing, allows
to apply patches over the source if needed, etc.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>