spi: tools: move to tools buildsystem

There is a nice buildsystem dedicated for userspace tools in Linux kernel tree.
Switch spi target to be built by it.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

tools/spi/Build

+spidev_test-y += spidev_test.o
+spidev_fdx-y += spidev_fdx.o

tools/spi/Makefile

+include ../scripts/Makefile.include
+
+ifeq ($(srctree),)
+srctree := $(patsubst %/,%,$(dir $(CURDIR)))
+srctree := $(patsubst %/,%,$(dir $(srctree)))
+endif
+
+# Do not use make's built-in rules
+# (this improves performance and avoids hard-to-debug behaviour);
+MAKEFLAGS += -r
+
 CC = $(CROSS_COMPILE)gcc
+LD = $(CROSS_COMPILE)ld
+CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include
+
+ALL_TARGETS := spidev_test spidev_fdx
+ALL_PROGRAMS := $(patsubst %,$(OUTPUT)%,$(ALL_TARGETS))
+
+all: $(ALL_PROGRAMS)
+
+export srctree OUTPUT CC LD CFLAGS
+include $(srctree)/tools/build/Makefile.include
 
-all: spidev_test spidev_fdx
+#
+# We need the following to be outside of kernel tree
+#
+$(OUTPUT)include/linux/spi/spidev.h: ../../include/uapi/linux/spi/spidev.h
+	mkdir -p $(OUTPUT)include/linux/spi 2>&1 || true
+	ln -sf $(CURDIR)/../../include/uapi/linux/spi/spidev.h $@
+
+prepare: $(OUTPUT)include/linux/spi/spidev.h
+
+#
+# spidev_test
+#
+SPIDEV_TEST_IN := $(OUTPUT)spidev_test-in.o
+$(SPIDEV_TEST_IN): prepare FORCE
+	$(Q)$(MAKE) $(build)=spidev_test
+$(OUTPUT)spidev_test: $(SPIDEV_TEST_IN)
+	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+
+#
+# spidev_fdx
+#
+SPIDEV_FDX_IN := $(OUTPUT)spidev_fdx-in.o
+$(SPIDEV_FDX_IN): prepare FORCE
+	$(Q)$(MAKE) $(build)=spidev_fdx
+$(OUTPUT)spidev_fdx: $(SPIDEV_FDX_IN)
+	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
 
 clean:
-	$(RM) spidev_test spidev_fdx
+	rm -f $(ALL_PROGRAMS)
+	rm -f $(OUTPUT)include/linux/spi/spidev.h
+	find $(if $(OUTPUT),$(OUTPUT),.) -name '*.o' -delete -o -name '\.*.d' -delete
+
+FORCE:
+
+.PHONY: all clean FORCE prepare