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/input/i8042.c b/drivers/input/i8042.c
index 0fd25b1..18476e9 100644
--- a/drivers/input/i8042.c
+++ b/drivers/input/i8042.c
@@ -274,7 +274,7 @@
 
 	/* Init keyboard device (default US layout) */
 	keymap = KBD_US;
-	penv = getenv("keymap");
+	penv = env_get("keymap");
 	if (penv != NULL) {
 		if (strncmp(penv, "de", 3) == 0)
 			keymap = KBD_GER;