termios: Separate flow control from normal handler
termios: Partially hide rtems_termios_tty

Move interrupt lock to device context and expose only this structure to
the read, write and set attributes device handler.  This makes these
device handler independent of the general Termios infrastructure
suitable for direct use in printk() support.
bsps: Add Termios console driver initialization

Add a simple Termios console driver using a table for statically
registered devices used in console_initialize() and dynamic installation
via console_device_install().
libchip/serial: Add alternative NS16550 driver

Use the Termios device API.
libchip/serial: Fix NS16550 for pppstart()

Call rtems_termios_dequeue_characters() only in case all characters of
the last write handler invocation have been transmitted.  This avoids
problems with pppstart() since the start line discipline does not know
how many characters have been dequeued and assumes the maximum.
libchip/serial: Task driven mode for NS16550
ppp: Nothing to transmit hint for Termios driver