env: allow to export only selected variables

New syntax:
	env export [-t | -b | -c] [-s size] addr [var ...]

With this change it is possible to provide a list of variables names
that shall be exported.  Whenno arguments are given, the whole
environment gets exported.

NOTE: The new handling of the "size" argument means a change to the
user API.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/include/search.h b/include/search.h
index b4edd43..ef53edb 100644
--- a/include/search.h
+++ b/include/search.h
@@ -91,7 +91,8 @@
 extern int hdelete_r(const char *__key, struct hsearch_data *__htab);
 
 extern ssize_t hexport_r(struct hsearch_data *__htab,
-		     const char __sep, char **__resp, size_t __size);
+		     const char __sep, char **__resp, size_t __size,
+		     int argc, char * const argv[]);
 
 extern int himport_r(struct hsearch_data *__htab,
 		     const char *__env, size_t __size, const char __sep,