Do not block the boot process for five seconds or longer. When
adding the configurable timeout the other timeout code got broken.
Remove it and instead supply uart_recv_bytes with the timeout per
charachter.
Before doing the NAND boot the 'I_ME' message will be sent and the
ubl will wait up to a second for a response. This interruption mode
can be disabled, e.g. when deploying the system in a publically
accessible system. Right now the possibility of easy recovery is
more important.