system: TARGET_GENERIC_GETTY: allow it to be disabled

For some systems, you don't want to run any getty, so allow the option
to be disabled when the empty string is used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

system/Config.in

@@ -145,6 +145,7 @@ config BR2_TARGET_GENERIC_GETTY_PORT
 	default "ttyS0"
 	help
 	  Specify a port to run a getty (login prompt) on.
+	  Set to the empty string to not run a getty.
 
 choice
 	prompt "Baudrate to use"

system/system.mk

@@ -49,12 +49,15 @@ endif
 
 TARGETS += target-root-passwd
 
+ifneq ($(TARGET_GENERIC_GETTY),)
 ifeq ($(BR2_ROOTFS_SKELETON_DEFAULT),y)
 ifeq ($(BR2_PACKAGE_SYSVINIT),y)
 TARGETS += target-generic-getty-sysvinit
 else
 TARGETS += target-generic-getty-busybox
 endif
+endif
+
 ifeq ($(BR2_TARGET_GENERIC_REMOUNT_ROOTFS_RW),y)
 TARGETS += target-generic-do-remount-rw
 else