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/arch/arm/mach-uniphier/mmc-first-dev.c b/arch/arm/mach-uniphier/mmc-first-dev.c
index 8c45229..acc859a 100644
--- a/arch/arm/mach-uniphier/mmc-first-dev.c
+++ b/arch/arm/mach-uniphier/mmc-first-dev.c
@@ -35,7 +35,7 @@
 	if (dev < 0)
 		return CMD_RET_FAILURE;
 
-	setenv_ulong("mmc_first_dev", dev);
+	env_set_ulong("mmc_first_dev", dev);
 	return CMD_RET_SUCCESS;
 }