Easy fix to flash openwrt direct from the webpage:
You take version 4 factory flash.
Do:
/openwrt/staging_dir/host/bin/mktplinkfw -H 0x07400005 -W 1 -F 4M -N OpenWrt -V r40718 -X 0x40000 -k /openwrt/build_dir/target-mips_34kc_uClibc-0.9.33.2/linux-ar71xx_generic/tmp/vmlinux-tl-wr740n-v4.bin.lzma -r /openwrt/build_dir/target-mips_34kc_uClibc-0.9.33.2/linux-ar71xx_generic/root.squashfs-raw -a 0x4 -j -o /openwrt/bin/ar71xx/openwrt-ar71xx-generic-tl-wr740n-v5-squashfs-factory.bin
You may need to change -r40718 to your version.
Now you will have openwrt-ar71xx-generic-tl-wr740n-v5-squashfs-factory.bin which you can flash directly from original web.
This is dirty way to do the things.
Someone must make patches in the trunk....