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>

Makefile.convert-dtsv0

@@ -5,7 +5,8 @@
 #
 
 CONVERT_SRCS = \
-	srcpos.c
+	srcpos.c \
+	util.c
 
 CONVERT_GEN_SRCS = convert-dtsv0-lexer.lex.c