Add an explicit "variant" field to the per-build manifest, sourced from
CMake's ${LABEL} variable which is already derived from the .px4board
filename stem (e.g. multicopter.px4board → "multicopter"). This is the
authoritative source of the variant name: CMake uses it everywhere else
in the tree for board selection.
Previously the variant was implicit in "target" (last underscore-
separated segment), forcing consumers to string-split and assume the
target always follows <board>_<variant>. Now it's a stable, machine-
readable field alongside target and label_pretty.
detect_firmware_category() now takes the variant as its first argument
instead of re-deriving it from target, removing a duplicated parse.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>
170b7b4102