rpi3: spawn getty for both HDMI and serial console

Spawn getty for both HDMI and serial console. Leverage
post-build.sh script for rpi3, which adds getty line for
tty1 in inittab [1].

[1]: https://patchwork.ozlabs.org/patch/602668/

Fixes: https://github.com/OP-TEE/build/issues/327


Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>