env: make himport_r() selective on variables

Add 2 new arguments to himport_r():

 o "nvars", "vars": number and list of variables to take into account
   (0 means ALL)

NOTE: This patch does not change the current behaviour.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/common/env_common.c b/common/env_common.c
index 911a6af..c5cefd8 100644
--- a/common/env_common.c
+++ b/common/env_common.c
@@ -196,7 +196,8 @@
 	}
 
 	if (himport_r(&env_htab, (char *)default_environment,
-			sizeof(default_environment), '\0', 0) == 0)
+			sizeof(default_environment), '\0', 0,
+			0, NULL) == 0)
 		error("Environment import failed: errno = %d\n", errno);
 
 	gd->flags |= GD_FLG_ENV_READY;
@@ -221,7 +222,8 @@
 		}
 	}
 
-	if (himport_r(&env_htab, (char *)ep->data, ENV_SIZE, '\0', 0)) {
+	if (himport_r(&env_htab, (char *)ep->data, ENV_SIZE, '\0', 0,
+			0, NULL)) {
 		gd->flags |= GD_FLG_ENV_READY;
 		return 1;
 	}