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/ti/am57xx/board.c b/board/ti/am57xx/board.c
index 126c9ef..bd94a90 100644
--- a/board/ti/am57xx/board.c
+++ b/board/ti/am57xx/board.c
@@ -592,7 +592,7 @@
 		/* we will let default be "no lcd" */
 	}
 out:
-	setenv("idk_lcd", idk_lcd);
+	env_set("idk_lcd", idk_lcd);
 	return;
 }
 
@@ -612,7 +612,7 @@
 	 * on HS devices.
 	 */
 	if (get_device_type() == HS_DEVICE)
-		setenv("boot_fit", "1");
+		env_set("boot_fit", "1");
 
 	/*
 	 * Set the GPIO7 Pad to POWERHOLD. This has higher priority