ar71xx: Make wget2nand fail if copying the kernel fails and use the correct file...

ar71xx: Make wget2nand fail if copying the kernel fails and use the correct file extension for the rootfs image

wget2nand didnt check the exit status after copying the kernel, if the
copying failed for some reason ( for example not enougs space on the
kernel partition) it simply continued extracting the rootfs.
I also changed the filename, which wget2nand trys to download (
.tar.gz instead of .tgz ).

Signed-off-by: Marko Foerster <mrkfoerster@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@21444 3c298f89-4303-0410-b956-a3cf2f4a3e73