fdt: Start a test for the fdt command

Add a basic test of the 'fdt addr' command, to kick things off.

This includes a new convenience function to run a command from a printf()
string.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/cmd/Makefile b/test/cmd/Makefile
index 4b2d7df..c331757 100644
--- a/test/cmd/Makefile
+++ b/test/cmd/Makefile
@@ -7,6 +7,7 @@
 endif
 obj-y += mem.o
 obj-$(CONFIG_CMD_ADDRMAP) += addrmap.o
+obj-$(CONFIG_CMD_FDT) += fdt.o
 obj-$(CONFIG_CMD_LOADM) += loadm.o
 obj-$(CONFIG_CMD_MEM_SEARCH) += mem_search.o
 obj-$(CONFIG_CMD_PINMUX) += pinmux.o