boards: arm: qemu_cortex_a9: move UART_CONSOLE

Move where the default value of `CONFIG_UART_CONSOLE` is set from
`Kconfig.defconfig` to `qemu_cortex_a9_defconfig`. This conforms to
the standard location and lets the default be overridden by
applications.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>