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>

cmd/jffs2.c

@@ -479,9 +479,9 @@ int do_jffs2_fsload(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	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];