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>

board/cadence/xtfpga/xtfpga.c

@@ -86,7 +86,7 @@ int misc_init_r(void)
 	 * Default MAC address comes from CONFIG_ETHADDR + DIP switches 1-6.
 	 */
 
-	char *s = getenv("ethaddr");
+	char *s = env_get("ethaddr");
 	if (s == 0) {
 		unsigned int x;
 		char s[] = __stringify(CONFIG_ETHBASE);