env: Rename common functions related to setenv()

We are now using an env_ prefix for environment functions. Rename these
commonly used functions, 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/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index c1985fd..aaa3a42 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -266,7 +266,7 @@
 	if (fdt_magic(fw_dtb_pointer) != FDT_MAGIC)
 		return;
 
-	setenv_hex("fdt_addr", fw_dtb_pointer);
+	env_set_hex("fdt_addr", fw_dtb_pointer);
 }
 
 /*