Specify minimum cmake version for generic_board.cmake

[lurch]

cmake/generic_board.cmake uses POP_FRONT, which https://cmake.org/cmake/help/latest/command/list.html#pop-front says was added in CMake version 3.15.
(this change means that people still using an ancient version of cmake will get a nicer error-message)

[kilograham]

This should probably go in the top-level CMakeLists.txt - i am loathe to change minimum versions, but then again, this code is always used an no one has complained, so most people must be >= 3.15

[lurch]

I wasn't sure what the "policy" was WRT CMake minimum versions (i.e. do we just have a single "global" minimum version, or does each CMakeLists.txt declare the minimum version that it needs itself?), so I based this change on https://github.com/raspberrypi/pico-sdk/blob/develop/tools/Findpicotool.cmake (which already declares a higher minimum-version than the top-level CMakeLists.txt).

this code is always used an no one has complained, so most people must be >= 3.15

I discovered this by trying to see (just out of curiosity) whether the SDK would still build on a Debian Buster docker image, which comes with CMake 3.13.4 (Debian Buster is EOL).

[kilograham]

@will-v-pi thoughts?

[will-v-pi]

FindPicotool.cmake is only used when the SDK needs to compile picotool (contrary to its name), so if you have installed picotool then that file won’t be used and a lower CMake version should work. I think it makes sense to do the same for generic_board.cmake as you could set your own simplified board cmake file if you need to use an older version of CMake.

I’d agree that for any CMakeLists.txt files we should update it in the top level project, but for these script-esque files which are not always included it’s probably better to put the minimum version in the file?