powerpc/virtex: Add uImage to the default images list

It is common to use U-Boot on Xilinx Virtex platforms.  This patch
ensures that CONFIG_DEFAULT_UIMAGE is selected for virtex

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/powerpc/platforms/40x/Kconfig

@@ -153,6 +153,7 @@ config 405GPR
 
 config XILINX_VIRTEX
 	bool
+	select DEFAULT_UIMAGE
 
 config XILINX_VIRTEX_II_PRO
 	bool