Config.in.legacy: add legacy entry for 4.20.x headers

Commit b3bba8e4254 (package/linux-headers: drop support 4.20 headers)
removed the option for 4.20.x kernel headers, but forgot to add a legacy
option.  Add that now.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in.legacy

@@ -153,6 +153,13 @@ config BR2_GDB_VERSION_8_0
 	  The 8.0.x version of gdb has been removed. Use a newer
 	  version instead.
 
+config BR2_KERNEL_HEADERS_4_20
+	bool "kernel headers version 4.20.x are no longer supported"
+	select BR2_LEGACY
+	help
+	  Version 4.20.x of the Linux kernel headers are no longer
+	  maintained upstream and are now removed.
+
 comment "Legacy options removed in 2019.05"
 
 config BR2_CSKY_DSP