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_ext.c b/common/spl/spl_ext.c
index f17c6b9..559ba0b 100644
--- a/common/spl/spl_ext.c
+++ b/common/spl/spl_ext.c
@@ -89,7 +89,7 @@
 		return -1;
 	}
 #if defined(CONFIG_SPL_ENV_SUPPORT)
-	file = getenv("falcon_args_file");
+	file = env_get("falcon_args_file");
 	if (file) {
 		err = ext4fs_open(file, &filelen);
 		if (err < 0) {
@@ -102,7 +102,7 @@
 			       file, err);
 			goto defaults;
 		}
-		file = getenv("falcon_image_file");
+		file = env_get("falcon_image_file");
 		if (file) {
 			err = spl_load_image_ext(spl_image, block_dev,
 						 partition, file);