core/debug: don't disable usart receiver

Otherwise reception on USART doesn't work and hence Ethersex
doesn't handle ECMDs via serial line.

This was changed by commit 1273083d59e45e1aa6ca9bbb22a19004bc9fcf48,
supposedly accidentaly.
fixed syntax error
Merge pull request #338 from AnDann/master

New module for BoilerSystemBus