Refactor in a similar way as done for rk3588 u-boot

• move docker image from bookworm to trixie
• use git_shallow_clone() from rk3588 u-boot script
• make script shellcheck clean
• indent with shfmt
• allow overriding CROSS_COMPILE environment variable from the outside
• make build() function very similar to rk3588 u-boot
• copy in defconfig for each platform and clean up afterwards
• use the machine name and the output of "git describe" in EXTRAVERSION
• print sha1sum for easier copypasting to reform-tools machine .conf files
• rename defconfig files to dtbnames
• Build linearly instead of using -j$(nproc)
• Avoid changing the working directory with cd and use "env --chdir=..."
• Fix the git SHA1 for all repositories.
• Perform a shallow git clone for all repositories.
• set CONFIG_DEFAULT_FDT_FILE so that ${fdtfile} is set in the u-boot environment

TODO:

• Test gitlab CI artifacts artifacts produced by MNT gitlab CI confirmed working

$ cat /proc/device-tree/chosen/u-boot,version
2023.10 MNT Reform 2 with LS1028A Module 2024-06-30-3-g9abf737c-g4459ed60-dirty