env: Rename getenv/_f() to env_get()

We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/net.c b/net/net.c
index be33c66..4f82a7e 100644
--- a/net/net.c
+++ b/net/net.c
@@ -319,7 +319,7 @@
 void net_auto_load(void)
 {
 #if defined(CONFIG_CMD_NFS)
-	const char *s = getenv("autoload");
+	const char *s = env_get("autoload");
 
 	if (s != NULL && strcmp(s, "NFS") == 0) {
 		/*
@@ -668,7 +668,7 @@
 	unsigned long retrycnt = 0;
 	int ret;
 
-	nretry = getenv("netretry");
+	nretry = env_get("netretry");
 	if (nretry) {
 		if (!strcmp(nretry, "yes"))
 			retry_forever = 1;
@@ -1538,5 +1538,5 @@
 
 ushort getenv_vlan(char *var)
 {
-	return string_to_vlan(getenv(var));
+	return string_to_vlan(env_get(var));
 }