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>

common/splash.c

@@ -60,7 +60,7 @@ __weak int splash_screen_prepare(void)
 #ifdef CONFIG_SPLASH_SCREEN_ALIGN
 void splash_get_pos(int *x, int *y)
 {
-	char *s = getenv("splashpos");
+	char *s = env_get("splashpos");
 
 	if (!s)
 		return;