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/boundary/nitrogen6x/nitrogen6x.c b/board/boundary/nitrogen6x/nitrogen6x.c
index c04926c..0e2c4fc 100644
--- a/board/boundary/nitrogen6x/nitrogen6x.c
+++ b/board/boundary/nitrogen6x/nitrogen6x.c
@@ -1021,6 +1021,6 @@
 #ifdef CONFIG_CMD_BMODE
 	add_board_boot_modes(board_boot_modes);
 #endif
-	setenv_hex("reset_cause", get_imx_reset_cause());
+	env_set_hex("reset_cause", get_imx_reset_cause());
 	return 0;
 }