rpi3: use ttyS0 for getty

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

rpi3.mk

@@ -8,6 +8,9 @@ override COMPILE_NS_KERNEL := 64
 override COMPILE_S_USER    := 64
 override COMPILE_S_KERNEL  := 64
 
+# Need to set this before including common.mk
+BUILDROOT_GETTY_PORT ?= ttyS0
+
 include common.mk
 
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