blackfin: Ignore generated uImages

We have the build infrastructure to generate uImages so we should ignore
the resulting generated files.
Signed-off-by: NMark Brown <broonie@linaro.org>
Acked-by: NMike Frysinger <vapier@gentoo.org>

arch/blackfin/boot/.gitignore

 vmImage*
 vmlinux*
+uImage*