binman: Tidy up pylint warnings in comp_util

Tweak some naming and comments to resolve these. Use WriteFile() to write
the file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index 5281dcd..b4323d5 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -1035,7 +1035,7 @@
         self.uncomp_data = indata
         if self.compress != 'none':
             self.uncomp_size = len(indata)
-        data = comp_util.Compress(indata, self.compress)
+        data = comp_util.compress(indata, self.compress)
         return data
 
     @classmethod