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/common/spl/spl_dfu.c b/common/spl/spl_dfu.c
index f5ec0fb..2c97473 100644
--- a/common/spl/spl_dfu.c
+++ b/common/spl/spl_dfu.c
@@ -40,7 +40,7 @@
 
 	/* set default environment */
 	set_default_env(0);
-	str_env = getenv(dfu_alt_info);
+	str_env = env_get(dfu_alt_info);
 	if (!str_env) {
 		error("\"dfu_alt_info\" env variable not defined!\n");
 		return -EINVAL;