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/net/bootp.c b/net/bootp.c
index 6c869ef..73370a1 100644
--- a/net/bootp.c
+++ b/net/bootp.c
@@ -504,7 +504,7 @@
 #endif
 
 	if (env_get("bootp_arch"))
-		clientarch = getenv_ulong("bootp_arch", 16, clientarch);
+		clientarch = env_get_ulong("bootp_arch", 16, clientarch);
 
 	if (clientarch > 0) {
 		*e++ = 93;	/* Client System Architecture */
diff --git a/net/net.c b/net/net.c
index 4f82a7e..0b87691 100644
--- a/net/net.c
+++ b/net/net.c
@@ -329,7 +329,7 @@
 		return;
 	}
 #endif
-	if (getenv_yesno("autoload") == 0) {
+	if (env_get_yesno("autoload") == 0) {
 		/*
 		 * Just use BOOTP/RARP to configure system;
 		 * Do not use TFTP to load the bootfile.