graphics/libyuv: fix with using git repo check
There is no subfolder called "libyuv" in libyuv's folder, check
libyuv/libyuv may trigger download libyuv in everytime, hence fixed by
check libyuv repo.
@@ -33,7 +33,7 @@ libyuv.tar.gz:
$(Q) patch -p0 < 0001-include-libyuv-fix-strict-prototype-warning.patch
# Download and unpack tarball if no git repo found
-ifeq ($(wildcard libyuv/libyuv),)
+ifeq ($(wildcard libyuv/.git),)
context:: libyuv.tar.gz
distclean::
