Implement and use an xstrdup() function

Many places in dtc use strdup(), but none of them actually check the
return value to see if the implied allocation succeeded.  This is a
potential bug, which we fix in the patch below by replacing strdup()
with an xstrdup() which in analogy to xmalloc() will quit with a fatal
error if the allocation fails.

I felt the introduciton of util.[ch] was a better choice
for utility oriented code than directly using srcpos.c
for the new string function.

This patch is a re-factoring of Dave Gibson's similar patch.

Signed-off-by: Jon Loeliger <jdl@freescale.com>

dtc.h

@@ -34,7 +34,10 @@
 #include <libfdt_env.h>
 #include <fdt.h>
 
+#include "util.h"
+
 #define DEFAULT_FDT_VERSION	17
+
 /*
  * Command line options
  */