Add XMEGA to the LUFA core library makefile's list of architectures, so that...

Add XMEGA to the LUFA core library makefile's list of architectures, so that the XMEGA files are properly cleaned on request.

LUFA/makefile

@@ -12,7 +12,7 @@
 # Check to see if the LUFA_PATH variable has not been set (the makefile is not being included from a project makefile)
 ifeq ($(origin LUFA_PATH), undefined)
    LUFA_ROOT_PATH = .
-   ARCH = {AVR8,UC3}
+   ARCH = {AVR8,UC3,XMEGA}
 else
    LUFA_ROOT_PATH = $(LUFA_PATH)/LUFA
 endif