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/common/bootm.c b/common/bootm.c
index 0c380e7..0856354 100644
--- a/common/bootm.c
+++ b/common/bootm.c
@@ -72,7 +72,7 @@
 		       char * const argv[])
 {
 	memset((void *)&images, 0, sizeof(images));
-	images.verify = getenv_yesno("verify");
+	images.verify = env_get_yesno("verify");
 
 	boot_start_lmb(&images);
 
@@ -534,7 +534,7 @@
 	 *	yes - we always fixup
 	 *	unset - we rely on the console silent flag
 	 */
-	want_silent = getenv_yesno("silent_linux");
+	want_silent = env_get_yesno("silent_linux");
 	if (want_silent == 0)
 		return;
 	else if (want_silent == -1 && !(gd->flags & GD_FLG_SILENT))