Rework the emulation of serial console via JTAG from simple ad-hoc
implementation of serial port routines to CONFIG_SERIAL_MULTI and
enable CONFIG_SERIAL_MULTI unconditionally for blackfin.

In order for the JTAG serial console to take precedence over all
other serial ports available in system, implement override for
default_serial_console call returning this JTAG serial console.

This brings in a bit of a growth of size, but eventually will allow
us to unconditionally enable CONFIG_SERIAL_MULTI throughout the whole
U-Boot and maintain only one serial subsystem.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Mike Frysinger <vapier@gentoo.org>