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/engicam/common/board.c b/board/engicam/common/board.c
index 56112d3..c7ec55f 100644
--- a/board/engicam/common/board.c
+++ b/board/engicam/common/board.c
@@ -21,7 +21,7 @@
 	char mmcblk[32];
 	u32 dev_no = mmc_get_env_dev();
 
-	setenv_ulong("mmcdev", dev_no);
+	env_set_ulong("mmcdev", dev_no);
 
 	/* Set mmcblk env */
 	sprintf(mmcblk, "/dev/mmcblk%dp2 rootwait rw", dev_no);