In order to handle other USB commands like GET_STATUS, SET_ATR
or GET_STATS, we introduce an intermediary queue. The USB receive
completion call-back puts the command on that intermediate 'main'
queue. From that intermediate queue, the are further
dispatched/processed according to their mesage type.
When the ISO7816 state machine requires us to transmit something, but
we don't have data ready to transmit, we should switch off TXRDY
interrupt generation.
What's missing is of course the other part that re-enabls TXRDY
generation once new data is available
We don't want to call into the bulk of card_emu.c from interrupt
context, so let's introduce a ring buffer as a softwre-FIFO between
the USART receiving a byte and the processing of that byte in
card_emu.c, which is then performed from the main loop outside
interrupt context.
