configs/qemu{x86, x86_64}: add a serial console

The current Buildroot defconfigs for qemu_x86 and qemu_x86_64
instantiate a console on tty1, which appears on QEMU's
graphical window. Add a console on the serial port (ttyS0) to
be used later for gitlab testing.

This change is need since the script used for gitlab testing
needs to use a serial output with pexpect.

This change is similar to the one made for raspberrypi [1] to
handle HDMI and serial console:

This requires three changes:
 1. have two 'console=' entries in the kernel command line: tty1,
    then ttyS0;
 2. change BR2_TARGET_GENERIC_GETTY_PORT to "console", so it starts
    a getty on the last console= passed to the kernel, ttyS0;
 3. add a new getty on tty1 to the generated inittab.

Step 2 is actually obtained by removing BR2_TARGET_GENERIC_GETTY_PORT
entirely from the defconfigs, since "console" is the default value.

Step 3 requires a post-build script since the Buildroot makefiles can
configure only one console.

Note: instead of simply adding a new getty on ttyS0 (which would
work) this patch actually changes BR2_TARGET_GENERIC_GETTY_PORT to
instantiate a console on UART, then adds back tty1 via
post-build.sh. This is done only to avoid the "GENERIC_SERIAL" comment
where we instantiate a console on QEMU graphical window, then
instantiate a really-serial console on another line.

The result is these two inittab lines:

  console::respawn:/sbin/getty -L  console 0 vt100 # GENERIC_SERIAL
  tty1::respawn:/sbin/getty -L  tty1 0 vt100 # QEMU graphical window

[1] 20878a10

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>