build.sh: refactor to avoid duplication

• replace proprietary x86_64 aml_encrypt_g12b executable with gxlimg
• 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 dts and defconfig for each platform and clean up afterwards
• use the machine name and the output of "git describe" in EXTRAVERSION
• rename defconfig files to dtbnames
• print sha1sum for easier copypaste into reform-tools machine.conf
• meson-g12b-bananapi-cm4-mnt-*_defconfig: set CONFIG_DEFAULT_FDT_FILE

TODO:

• test on a311d Artifacts produced by MNT gitlab CI confirmed working on A311D classic Reform

$ cat /proc/device-tree/chosen/u-boot,version
2024.04 MNT Reform 2 with BPI-CM4 Module 2024-12-23-5-g11a2a19-g25049ad5-dirty