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/arch/arm/mach-imx/video.c b/arch/arm/mach-imx/video.c
index 55242f0..c670c5d 100644
--- a/arch/arm/mach-imx/video.c
+++ b/arch/arm/mach-imx/video.c
@@ -10,7 +10,7 @@
 {
 	int i;
 	int ret;
-	char const *panel = getenv("panel");
+	char const *panel = env_get("panel");
 
 	if (!panel) {
 		for (i = 0; i < display_count; i++) {