env: Rename common functions related to setenv()

We are now using an env_ prefix for environment functions. Rename these
commonly used functions, for consistency. Also add function comments in
common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/ubifs/ubifs.c b/fs/ubifs/ubifs.c
index db29489..8f1c9d1 100644
--- a/fs/ubifs/ubifs.c
+++ b/fs/ubifs/ubifs.c
@@ -941,7 +941,7 @@
 
 	err = ubifs_read(filename, (void *)(uintptr_t)addr, 0, size, &actread);
 	if (err == 0) {
-		setenv_hex("filesize", actread);
+		env_set_hex("filesize", actread);
 		printf("Done\n");
 	}