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/jffs2.c b/cmd/jffs2.c
index add89de..aee2f45 100644
--- a/cmd/jffs2.c
+++ b/cmd/jffs2.c
@@ -479,9 +479,9 @@
 	ulong offset = load_addr;
 
 	/* pre-set Boot file name */
-	if ((filename = getenv("bootfile")) == NULL) {
+	filename = env_get("bootfile");
+	if (!filename)
 		filename = "uImage";
-	}
 
 	if (argc == 2) {
 		filename = argv[1];