dtc: Simpler interface to source file management

This patch cleans up our handling of input files, particularly dts
source files, but also (to an extent) other input files such as those
used by /incbin/ and those used in -I dtb and -I fs modes.

We eliminate the current clunky mechanism which combines search paths
(which we don't actually use at present) with the open relative to
current source file behaviour, which we do.

Instead there's a single srcfile_relative_open() entry point for
callers which opens a new input file relative to the current source
file (which the srcpos code tracks internally).  It doesn't currently
do search paths, but we can add that later without messing with the
callers, by drawing the search path from a global (which makes sense
anyway, rather than shuffling it around the rest of the processing
code).

That suffices for non-dts input files.  For the actual dts files,
srcfile_push() and srcfile_pop() wrappers open the file while also
keeping track of it as the current source file for future opens.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -28,3 +28,27 @@ char *xstrdup(const char *s)
 
 	return dup;
 }
+
+char *join_path(const char *path, const char *name)
+{
+	int lenp = strlen(path);
+	int lenn = strlen(name);
+	int len;
+	int needslash = 1;
+	char *str;
+
+	len = lenp + lenn + 2;
+	if ((lenp > 0) && (path[lenp-1] == '/')) {
+		needslash = 0;
+		len--;
+	}
+
+	str = xmalloc(len);
+	memcpy(str, path, lenp);
+	if (needslash) {
+		str[lenp] = '/';
+		lenp++;
+	}
+	memcpy(str+lenp, name, lenn+1);
+	return str;
+}