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/api/api.c b/api/api.c
index 23fa177..7eee2fc 100644
--- a/api/api.c
+++ b/api/api.c
@@ -458,7 +458,7 @@
 	if ((value = (char **)va_arg(ap, uintptr_t)) == NULL)
 		return API_EINVAL;
 
-	*value = getenv(name);
+	*value = env_get(name);
 
 	return 0;
 }