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/cmd/gpt.c b/cmd/gpt.c
index fbfbe7b..638aa19 100644
--- a/cmd/gpt.c
+++ b/cmd/gpt.c
@@ -55,14 +55,14 @@
 	memset(s + strlen(s) - 1, '\0', 1);
 	memmove(s, s + 2, strlen(s) - 1);
 
-	e = getenv(s);
+	e = env_get(s);
 	if (e == NULL) {
 #ifdef CONFIG_RANDOM_UUID
 		debug("%s unset. ", str);
 		gen_rand_uuid_str(uuid_str, UUID_STR_FORMAT_GUID);
 		env_set(s, uuid_str);
 
-		e = getenv(s);
+		e = env_get(s);
 		if (e) {
 			debug("Set to random.\n");
 			ret = 0;