Ignore those scripts. They are intended to bootstrap Debian for Turris Omnia not Mox. For Turris Omnia it has to compile additional tools and primarily kernel. For Mox it should be for basic functionality enough to use upstream kernel. It uses u-boot and it contains distro-boot environment so it can chainboot bootloader such as syslinux (https://github.com/u-boot/u-boot/blob/master/doc/README.distro). Turris MOX also has device tree in flash on board so you do not have to worry about that either.
Another options is to use u-boot directly and boot your distribution that way.
We have plans to create documentation on how to boot standard ditribution (probably specific one such as OpenSUSE) but currently we are occupied with Turris OS and MOX in general.
In general Mox is same as any aarch64 processor board and so if you have experience on how to boot system on those then you should not have any trouble.
If you are using UART then note that you need 1.8V serial convertor on MOX. Using 5V is going to end up in smoke…