env: Rename getenv_hex(), getenv_yesno(), getenv_ulong()

We are now using an env_ prefix for environment functions. Rename these
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 7bcbc31..7e7056c 100644
--- a/board/ti/am57xx/board.c
+++ b/board/ti/am57xx/board.c
@@ -743,7 +743,7 @@
 #ifdef CONFIG_SPL_ENV_SUPPORT
 	env_init();
 	env_load();
-	if (getenv_yesno("boot_os") != 1)
+	if (env_get_yesno("boot_os") != 1)
 		return 1;
 #endif