imgtool: Move keys to submodule

Move this code into a submodule to prepare to cleanup and enhance the
key management code.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/scripts/imgtool/keys.py b/scripts/imgtool/keys/__init__.py
similarity index 100%
rename from scripts/imgtool/keys.py
rename to scripts/imgtool/keys/__init__.py