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/arch/arm/mach-imx/mx7/soc.c b/arch/arm/mach-imx/mx7/soc.c
index 4cf977e..7152c84 100644
--- a/arch/arm/mach-imx/mx7/soc.c
+++ b/arch/arm/mach-imx/mx7/soc.c
@@ -256,9 +256,9 @@
 {
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
 	if (is_mx7d())
-		setenv("soc", "imx7d");
+		env_set("soc", "imx7d");
 	else
-		setenv("soc", "imx7s");
+		env_set("soc", "imx7s");
 #endif
 
 	return 0;