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/varisys/common/sys_eeprom.c b/board/varisys/common/sys_eeprom.c
index b55ab81..d235897 100644
--- a/board/varisys/common/sys_eeprom.c
+++ b/board/varisys/common/sys_eeprom.c
@@ -401,7 +401,7 @@
 			mac[5]);
 
 		printf("MAC: %s\n", ethaddr);
-		setenv(envvar, ethaddr);
+		env_set(envvar, ethaddr);
 	}
 
 	return ret;
@@ -487,7 +487,7 @@
 			 * (i.e. have not yet been set)
 			 */
 			if (!getenv(enetvar))
-				setenv(enetvar, ethaddr);
+				env_set(enetvar, ethaddr);
 		}
 	}