kconfig: expand file names

This will allow to use the following construct in source files:

config FOO
    string
    default "foo"

source "$FOO/Kconfig"
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Michal Marek <mmarek@suse.cz>

scripts/kconfig/util.c

@@ -12,15 +12,18 @@
 struct file *file_lookup(const char *name)
 {
 	struct file *file;
+	const char *file_name = sym_expand_string_value(name);
 
 	for (file = file_list; file; file = file->next) {
-		if (!strcmp(name, file->name))
+		if (!strcmp(name, file->name)) {
+			free((void *)file_name);
 			return file;
+		}
 	}
 
 	file = malloc(sizeof(*file));
 	memset(file, 0, sizeof(*file));
-	file->name = strdup(name);
+	file->name = file_name;
 	file->next = file_list;
 	file_list = file;
 	return file;