rescue-image: fix compilation
It fixes issue: cp: cannot stat '/home/beast/beast/workspace/turris-os-packages-dragons-omnia/build/target/linux/mvebu/files-5.4/.': No such file or directory
By looking at OpenWrt repository: https://github.com/openwrt/openwrt/tree/openwrt-19.07/target/linux/mvebu I see there is folder files/arch , it is no longer based on kernel patch version as it was used in OpenWrt 19.07.
For reference: https://github.com/openwrt/openwrt/tree/openwrt-19.07/target/linux/mvebu
Fixes: https://gitlab.nic.cz/turris/turris-os-packages/-/issues/636