scripts/ipkg-build: quash error messages when conffiles.resolved is empty

scripts/ipkg-build: quash error messages when conffiles.resolved is empty

When Package/xx/conffiles only contains directories that are empty at
package time, conffiles.resolved will be missing and the following
error messages will appear in the build log.

	/home/yousong/git-repo/openwrt/openwrt/scripts/ipkg-build -c -o 0 -g 0 /home/yousong/git-repo/openwrt/openwrt/build_dir/target-mips_24kc_musl/linux-malta_be/openvswitch-2.10.0/ipkg-mips_24kc/openvswitch-common /home/yousong/git-repo/openwrt/openwrt/bin/packages/mips_24kc/packages
	mv: cannot stat 'CONTROL/conffiles.resolved': No such file or directory
	chmod: cannot access 'CONTROL/conffiles': No such file or directory

It will not break the ipkg-build process.  The change is merely cosmetic
to not cause confusion when reading logs

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
(backported from 9e58c20ec9d48ee41a4a306b0cd6d3aece285efd)