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

• move docker image from bookworm to trixie
• cherry-pick patches from u-boot upstream:
  • change pylibfdt from distutils (deprecated) to setuptools
  • disable version normalization to prevent exceptions when using the machine name in the EXTRAVERSION string
  • fix swig for versions later than 4.3.0
• move code from .gitlab-ci.yml to build.sh
• 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 dts and defconfig for each platform and clean up afterwards
• use the machine name and the output of "git describe" in EXTRAVERSION

TODO:

• needs to get tested

EDIT:

I successfully tested the artifact produced by the pipeline on imx8m+ with classic Reform:

U-Boot 2022.04 MNT Reform 2 with i.MX8MP Module 2025-01-12-3-g54ab337-gb0e908b1-dirty (Nov 03 2025 - 10:29:45 +0000)