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/amlogic/odroid-c2/odroid-c2.c b/board/amlogic/odroid-c2/odroid-c2.c
index b29f56d..f2a394a 100644
--- a/board/amlogic/odroid-c2/odroid-c2.c
+++ b/board/amlogic/odroid-c2/odroid-c2.c
@@ -55,7 +55,7 @@
 		len = meson_sm_read_efuse(EFUSE_SN_OFFSET, serial,
 			EFUSE_SN_SIZE);
 		if (len == EFUSE_SN_SIZE) 
-			setenv("serial#", serial);
+			env_set("serial#", serial);
 	}
 
 	return 0;