U-Boot build dependencies are listed on their online documentation [1],
but the listed Debian packages also include test-only dependencies.
While installing dependencies, install the packages necessary to build
U-Boot, except for the test-only ones I could identify.

[1] https://u-boot.readthedocs.io/en/latest/build/gcc.html

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

python2 is eol and the only thing that needed it was build scripts
inside tianocore, back in osbmk days when tianocore was supported
in the (osboot) build system. nothing else requires it, so chuck it

osboot is now part of libreboot, and will soon shut down.
libreboot now conforms to osboot policy.