atmel mirrors are only needed for atmel targets

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

target/device/Config.in.mirrors

@@ -64,7 +64,7 @@ config BR2_DEBIAN_MIRROR
 
 	  Usually, just add your country code like XX here:
 	  http://ftp.XX.debian.org
-
+if BR2_TARGET_ATMEL
 source	"target/device/Atmel/Config.in.mirrors"
-
+endif
 endmenu