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/drivers/video/cfb_console.c b/drivers/video/cfb_console.c
index b6fc7e1..6c5425c 100644
--- a/drivers/video/cfb_console.c
+++ b/drivers/video/cfb_console.c
@@ -1856,7 +1856,7 @@
 	splash_get_pos(&video_logo_xpos, &video_logo_ypos);
 
 #ifdef CONFIG_SPLASH_SCREEN
-	s = getenv("splashimage");
+	s = env_get("splashimage");
 	if (s != NULL) {
 		ret = splash_screen_prepare();
 		if (ret < 0)