ci/broadcom: allow custom kernels

ci/broadcom: allow custom kernels

So far, testing VC4 and V3D/V3DV requires the CI runners having access
to a Raspberry Pi 3/4 kernel, and the correspondent modules and
bootloader files. If a different kernel must be used, it means touching
the runners to provide them.

This commit adds the option to define an URL pointing to a (compressed)
tarball containing such files, without requiring dealing with the
runners. This link is provided through the `BM_BOOTFS` job variable.

The tarball must contain two directories in the root: a `/boot`
directory (containing the kernel, DTBs and bootloader files), and a
`/lib/modules` (or `/usr/lib/modules`) with the kernel modules.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9527>