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/board/freescale/common/sys_eeprom.c b/board/freescale/common/sys_eeprom.c
index 29aa778..c593cff 100644
--- a/board/freescale/common/sys_eeprom.c
+++ b/board/freescale/common/sys_eeprom.c
@@ -479,7 +479,7 @@
 			 * (i.e. have not yet been set)
 			 */
 			if (!getenv(enetvar))
-				setenv(enetvar, ethaddr);
+				env_set(enetvar, ethaddr);
 		}
 	}