[IMP] hw_scanner: support multiple simultaneously attached devices

The hw_scanner module not only supports barcode readers (although that
was it's original intention). It also happens to support certain card
readers (like the MagTek Dynamag we use for Mercury) because they act
the same way as barcode scanners (like a keyboard and end sequence with
ENTER).

Before we supported Mercury there was no real reason to support multiple
devices on the POSBox, because it didn't make much sense to attach >1
barcode reader to the same POSBox. With Mercury however, this is a real
use case, as users want both a barcode reader and a card reader.

This commit implements support for this, while keeping the functionality
as close as possible to how it was before (eg. care was taken to not
break plug and play support).

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