Convert mk-s390-cdboot to python3 (#1497141)

It was pulling in py2 along with py3. Fix #! and make sure code works
with py3.

src/bin/mk-s390-cdboot

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 #
 # mk-s390-cdboot
 #
@@ -17,8 +17,6 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #
-from __future__ import print_function
-
 import argparse
 import os
 import shutil
@@ -96,7 +94,8 @@ def main():
         if not os.path.exists(f):
             errors.append("ERROR: %s is missing" % f)
     if errors:
-        map(print, errors)
+        for e in errors:
+            print(e)
         sys.exit(1)
 
     print("Creating bootable CD-ROM image...")