dont run Fibre tup rules by default

Building the Fibre test binaries is distracting when it's just used as a library in another project. Now it is only built if CONFIG_BUILD_FIBRE_TESTS=true

Firmware/fibre/cpp/package.lua

@@ -0,0 +1,8 @@
+
+tup.include('../tupfiles/build.lua')
+
+fibre_package = define_package{
+    sources={'protocol.cpp', 'posix_tcp.cpp', 'posix_udp.cpp'},
+    libs={'pthread'},
+    headers={'include'}
+}