env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/qfw.c b/cmd/qfw.c
index 12436ec..9b12ab4 100644
--- a/cmd/qfw.c
+++ b/cmd/qfw.c
@@ -55,7 +55,7 @@
 		 * when invoking qemu), do not update bootargs
 		 */
 		if (*data_addr != '\0') {
-			if (setenv("bootargs", data_addr) < 0)
+			if (env_set("bootargs", data_addr) < 0)
 				printf("warning: unable to change bootargs\n");
 		}
 	}