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/cmd/ximg.c b/cmd/ximg.c
index 50dc00e..21b5c37 100644
--- a/cmd/ximg.c
+++ b/cmd/ximg.c
@@ -52,7 +52,7 @@
 #endif
 	uint8_t		comp;
 
-	verify = getenv_yesno("verify");
+	verify = env_get_yesno("verify");
 
 	if (argc > 1) {
 		addr = simple_strtoul(argv[1], NULL, 16);