imgtool: Fix 'custom_tlvs' argument handling

When Image.create() is called without the 'custom_tlvs' argument, it
gets its default value (None). It must be checked before performing
any operations on/with it.

Change-Id: I8e0755265f35f9eeb796fe078a6ad8c8d9f2b8da
Signed-off-by: David Vincze <david.vincze@linaro.org>