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_fat.c b/common/spl/spl_fat.c
index 5e31216..60b85f0 100644
--- a/common/spl/spl_fat.c
+++ b/common/spl/spl_fat.c
@@ -113,7 +113,7 @@
 		return err;
 
 #if defined(CONFIG_SPL_ENV_SUPPORT) && defined(CONFIG_SPL_OS_BOOT)
-	file = getenv("falcon_args_file");
+	file = env_get("falcon_args_file");
 	if (file) {
 		err = file_fat_read(file, (void *)CONFIG_SYS_SPL_ARGS_ADDR, 0);
 		if (err <= 0) {
@@ -121,7 +121,7 @@
 			       file, err);
 			goto defaults;
 		}
-		file = getenv("falcon_image_file");
+		file = env_get("falcon_image_file");
 		if (file) {
 			err = spl_load_image_fat(spl_image, block_dev,
 						 partition, file);