boards.txt.py: fix extra newline at the end of JSON output

Author: igrr

package/package_esp8266com_index.template.json

@@ -261,4 +261,3 @@
     }
   ]
 }
-

tools/boards.txt.py

@@ -1190,14 +1190,10 @@ def package ():
     newfilestr = re.sub(r'"boards":[^\]]*\],', substitution, filestr, re.MULTILINE)
 
     if packagegen:
-        realstdout = sys.stdout
-        sys.stdout = open(pkgfname, 'w')
-
-    print newfilestr
-
-    if packagegen:
-        sys.stdout.close()
-        sys.stdout = realstdout
+        with open(pkgfname, 'w') as package_file:
+            package_file.write(newfilestr)
+    else:
+        sys.stdout.write(newfilestr)
 
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