modernize u-boot build using lessons earned from rk3588 u-boot

• .gitlab-ci.yml: switch to Debian Trixie
• board/boundary/nitrogen8m_som/spl.c: cast memory address to pointer in hexdump() call fixing -Wint-conversion error
• build.sh: allow setting CROSS_COMPILE from the outside
• build.sh: adapt the EXTRAVERSION format used by rk3588 u-boot
• build.sh: set ${fdtfile} to hdmi and non-hdmi dtb depending on the board
• build.sh: print sha1sum for easier copypasting to reform-tools machine .conf files

TODO:

• tested successfully on imx8mq
• investigate why /proc/device-tree/chosen/u-boot,version is missing. u-boot too old? Done: backported commit 622ecee93a604c017aca23906c5cc903b4002b78 from u-boot upstream