wolbBoot CMake Downloads

Device-specific supplemental download specifications.

Include in CmakePresets.json via the WOLFBOOT_DOWNLOADS_CMAKE

"cacheVariables": {
  "WOLFBOOT_DOWNLOADS_CMAKE": "${sourceDir}/cmake/downloads/my_dependency.cmake"
}

Format for my_dependency.cmake entries:

add_download(
    NAME some_name
    URL  https://github.com/some/repo.git
    TAG  5.9.0
)