Python 3 compat.

Try to keep our Python sources compatible with both Python 2.7 and 3.

Check with 'pylint --py3k' and 'python -3'.

meta/isa/riscv/settings.py

@@ -1,11 +1,11 @@
 """
 RISC-V settings.
 """
-
+from __future__ import absolute_import
 from cretonne import SettingGroup, BoolSetting
 from cretonne.predicates import And
 import cretonne.settings as shared
-from defs import isa
+from .defs import isa
 
 isa.settings = SettingGroup('riscv', parent=shared.group)