dm: Don't run tests if U-Boot cannot be built

There is no point in running the tests if U-Boot cannot be built. Abort in
this case.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/test-dm.sh

 #!/bin/sh
 
+die() {
+	echo $1
+	exit 1
+}
+
 NUM_CPUS=$(cat /proc/cpuinfo |grep -c processor)
 dtc -I dts -O dtb test/dm/test.dts -o test/dm/test.dtb
-make O=sandbox sandbox_config
-make O=sandbox -s -j${NUM_CPUS}
+make O=sandbox sandbox_config || die "Cannot configure U-Boot"
+make O=sandbox -s -j${NUM_CPUS} || die "Cannot build U-Boot"
 dd if=/dev/zero of=spi.bin bs=1M count=2
 ./sandbox/u-boot -d test/dm/test.dtb -c "dm test"
 rm spi.bin