make xz ramdisk images also available for 2.6.39+

2.6.38 isn't the only kernel supporting it.

SVN-Revision: 31343

Config.in

@@ -46,7 +46,7 @@ menu "Target Images"
 
 			config TARGET_INITRAMFS_COMPRESSION_XZ
 				bool "xz"
-				depends LINUX_2_6_38
+				depends !LINUX_2_6_30&&!LINUX_2_6_31&&!LINUX_2_6_32&&!LINUX_2_6_36&&!LINUX_2_6_37
 		endchoice
 
 		config EXTERNAL_CPIO