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/board/syteco/zmx25/zmx25.c b/board/syteco/zmx25/zmx25.c
index e02dcf2..85fda13 100644
--- a/board/syteco/zmx25/zmx25.c
+++ b/board/syteco/zmx25/zmx25.c
@@ -146,7 +146,7 @@
 	udelay(5000);
 #endif
 
-	e = getenv("gs_base_board");
+	e = env_get("gs_base_board");
 	if (e != NULL) {
 		if (strcmp(e, "G283") == 0) {
 			int key = gpio_get_value(IMX_GPIO_NR(2, 29));