FORCE_KERNEL_TAG allows testing kernel uprevs without rebuilding
containers by supplying an external kernel directly for booting on
hardware devices. Renaming it to EXTERNAL_KERNEL_TAG clarifies its
purpose, distinguishing it from KERNEL_TAG which rebuilds containers.
Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31795>