cygwin fix

target/config/Config.in.tools

@@ -34,6 +34,7 @@ config ADK_HOST_BUILD_LIBTOOL
 config ADK_HOST_BUILD_U_BOOT
 	boolean
 	default n if ADK_HOST_DARWIN
+	default n if ADK_HOST_CYGWIN
 	default y
 
 # always required, but can be provided by host