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/log.c b/cmd/log.c
index 873ee40..7a3bd5c 100644
--- a/cmd/log.c
+++ b/cmd/log.c
@@ -71,7 +71,8 @@
 #endif
 
 	/* Set up log version */
-	if ((s = getenv ("logversion")) != NULL)
+	s = env_get("logversion");
+	if (s)
 		log_version = (int)simple_strtoul(s, NULL, 10);
 
 	if (log_version == 2)
@@ -94,7 +95,8 @@
 		log->v2.start = log->v2.con;
 
 	/* Initialize default loglevel if present */
-	if ((s = getenv ("loglevel")) != NULL)
+	s = env_get("loglevel");
+	if (s)
 		console_loglevel = (int)simple_strtoul(s, NULL, 10);
 
 	gd->flags |= GD_FLG_LOGINIT;