描述：编译为userdebug版本时调试时，调试串口是可以正常输入输出的，编译为user版本时，串口就只能输出不能输入了。
原因：
out/target/product/rk3288/recovery/root/init.rc
# Always start adbd on userdebug and eng builds
on property:ro.debuggable=1
write /sys/class/android_usb/android0/enable 1
start adbd
start console
可见，console的启用受ro.debuggable的控制，即是ro.debuggable=1时，才启动console。
可以通过指令getprop ro.debuggable查看该属性的值：
该属性在build/core/main.mk中定义：
ifeq (true,$(strip $(enable_target_debugging)))
# Target is more debuggable and adbd is on by default
ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=1
# Enable Dalvik lock contention logging.
ADDITIONAL_BUILD_PROPERTIES += dalvik.vm.lockprof.threshold=500
# Include the debugging/testing OTA keys in this build.
INCLUDE_TEST_OTA_KEYS := true
else # !enable_target_debugging
# Target is less debuggable and adbd is off by default
ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=0
endif # !enable_target_debugging
解决：在main.mk中修改ro.debuggable=1
