kernel: Error on duplicate modules

This can happen when an out of tree module target uses a matching module
name from the base kernel. This causes two problems:

1) Depmod gets confused and only pulls symbols from one of the modules
2) Copying the modules to modules_out is not entirely deterministic and
   is based on the first variant to be returned by find, meaning that
   the variant that gets copied to the device could change build to
   build.

To avoid these issues, fail the build if this happens and force the
build target to only generate one copy of each module name.

Change-Id: I2e47ba4e142054feabaa1ab80fbbe0332fd84a62