UART: Implement a system of flow control

UART: Implement a system of flow control

+ The uart now signals (using IRQs) when it's fifo is full and
empty. This allow controling code to send new bytes, or to pause.
+ The uart also now understand the Baud rate and prints it.
+ Added new IOCTL to get/set the UART emulation flags

A new mode of the UART was made specialy for code that spinloops
waiting to get/send a byte. Now if the uart detects that, it will
insert a usleep() when the uart is idle, this will prevent
code from eating 100% cpu doing nothing.

Signed-off-by: Michel Pollet <buserror@gmail.com>