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/autoboot.c b/common/autoboot.c
index 35c1e97..a011865 100644
--- a/common/autoboot.c
+++ b/common/autoboot.c
@@ -301,7 +301,7 @@
 	bootcount++;
 	bootcount_store(bootcount);
 	env_set_ulong("bootcount", bootcount);
-	bootlimit = getenv_ulong("bootlimit", 10, 0);
+	bootlimit = env_get_ulong("bootlimit", 10, 0);
 #endif /* CONFIG_BOOTCOUNT_LIMIT */
 
 	s = env_get("bootdelay");