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>
diff --git a/flattree.c b/flattree.c
index 76acd28..50190b8 100644
--- a/flattree.c
+++ b/flattree.c
@@ -601,7 +601,7 @@
 		len++;
 	} while ((*p++) != '\0');
 
-	str = strdup(inb->ptr);
+	str = xstrdup(inb->ptr);
 
 	inb->ptr += len;
 
@@ -643,7 +643,7 @@
 		p++;
 	}
 
-	return strdup(inb->base + offset);
+	return xstrdup(inb->base + offset);
 }
 
 static struct property *flat_read_property(struct inbuf *dtbuf,
@@ -710,7 +710,7 @@
 	if (!streq(ppath, "/"))
 		plen++;
 
-	return strdup(cpath + plen);
+	return xstrdup(cpath + plen);
 }
 
 static struct node *unflatten_tree(struct inbuf *dtbuf,