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/common/env_sf.c b/common/env_sf.c
index d3b36d0..a1ff297 100644
--- a/common/env_sf.c
+++ b/common/env_sf.c
@@ -91,7 +91,7 @@
 	}
 
 	res = (char *)&env_new.data;
-	len = hexport_r(&env_htab, '\0', &res, ENV_SIZE);
+	len = hexport_r(&env_htab, '\0', &res, ENV_SIZE, 0, NULL);
 	if (len < 0) {
 		error("Cannot export environment: errno = %d\n", errno);
 		return 1;
@@ -293,7 +293,7 @@
 	}
 
 	res = (char *)&env_new.data;
-	len = hexport_r(&env_htab, '\0', &res, ENV_SIZE);
+	len = hexport_r(&env_htab, '\0', &res, ENV_SIZE, 0, NULL);
 	if (len < 0) {
 		error("Cannot export environment: errno = %d\n", errno);
 		goto done;