target/generic: support for busybox >= 1.14.x speed=0 handling

target/generic/Config.in

@@ -163,6 +163,8 @@ choice
 	help
 	  Select a baudrate to use.
 
+config BR2_TARGET_GENERIC_GETTY_BAUDRATE_KEEP
+       bool "keep kernel default"
 config BR2_TARGET_GENERIC_GETTY_BAUDRATE_9600
 	bool "9600"
 config BR2_TARGET_GENERIC_GETTY_BAUDRATE_19200
@@ -177,6 +179,7 @@ endchoice
 
 config BR2_TARGET_GENERIC_GETTY_BAUDRATE
 	string
+	default "0"		if BR2_TARGET_GENERIC_GETTY_BAUDRATE_KEEP
 	default "9600"		if BR2_TARGET_GENERIC_GETTY_BAUDRATE_9600
 	default "19200"		if BR2_TARGET_GENERIC_GETTY_BAUDRATE_19200
 	default "38400"		if BR2_TARGET_GENERIC_GETTY_BAUDRATE_38400