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>
diff --git a/common.mk b/common.mk
index 0c24c7e..48c31d4 100644
--- a/common.mk
+++ b/common.mk
@@ -207,6 +207,10 @@
 ifneq (,$(BR2_ROOTFS_OVERLAY))
 	@echo "BR2_ROOTFS_OVERLAY=\"$(BR2_ROOTFS_OVERLAY)\"" >> ../out-br/extra.conf
 endif
+ifneq (,$(BR2_ROOTFS_POST_BUILD_SCRIPT))
+	@echo "BR2_ROOTFS_POST_BUILD_SCRIPT=\"$(BR2_ROOTFS_POST_BUILD_SCRIPT)\"" >> \
+		../out-br/extra.conf
+endif
 	@echo "BR2_PACKAGE_OPTEE_TEST_CROSS_COMPILE=\"$(CROSS_COMPILE_S_USER)\"" >> \
 		../out-br/extra.conf
 	@echo "BR2_PACKAGE_OPTEE_EXAMPLES_CROSS_COMPILE=\"$(CROSS_COMPILE_S_USER)\"" >> \