This commit fixes the linking issues in the RP2 port that arise when
explicitly disabling USB CDC support.
The console input ringbuffer needs to be made available if dupterm
support is enabled, and the console character input function would
always attempt to read from the input ringbuffer even if CDC support is
disabled and the console isn't bound to any UART.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>