ci: Introduce CI_HOST_* and CI_TARGET_*

Replace CI_SYSTEM_NAME and CI_MACHINE_NAME with CI_HOST_SYSTEM and
CI_HOST_MACHINE, respectively.

Introduce CI_TARGET_SYSTEM and CI_TARGET_MACHINE, defaulting to
CI_HOST_SYSTEM and CI_HOST_MACHINE, respectively.

Use CI_TARGET_SYSTEM and CI_TARGET_MACHINE in the naming scheme of
target build and install directories, in order to allow concurrent
verification builds for multiple cross-platform targets.
4 files changed