rpi3: set BR2_TARGET_GENERIC_GETTY_PORT

In commit (7fbd6ce22a) "common.mk: buildroot: append BR2_ variables to
config automatically", flags were refactored and the old
BUILDROOT_GETTY_PORT got replaced with BR2_TARGET_GENERIC_GETTY_PORT.
Because of that Rpi3 didn't spawn a console when booting up. By setting
BR2_TARGET_GENERIC_GETTY_PORT to ttyS0 things will work again.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (Rpi3)
Reviewed-by: Jerome Forissier <jerome@forissier.org>
diff --git a/rpi3.mk b/rpi3.mk
index 70b14df..c333393 100644
--- a/rpi3.mk
+++ b/rpi3.mk
@@ -9,7 +9,7 @@
 override COMPILE_S_KERNEL  := 64
 
 # Need to set this before including common.mk
-BUILDROOT_GETTY_PORT ?= ttyS0
+BR2_TARGET_GENERIC_GETTY_PORT ?= ttyS0
 BR2_ROOTFS_POST_BUILD_SCRIPT ?= "board/raspberrypi3-64/post-build.sh"
 
 include common.mk