crossystem: Make crossystem vendor_available

Making crossystem `vendor_available` instead of `vendor` means it can be
installed in /system/bin/ or /vendor/bin/ depending on which target is
used, `crossystem` or `crossystem.vendor` respectively.

BUG=b:374130759
TEST=Build
BRANCH=None

Change-Id: I9a27f92c975b23d95f9e8dbb9d2189a1cd7518d2
Signed-off-by: Rob Barnes <robbarnes@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/5979089
Tested-by: Jakub Czapiga <czapiga@google.com>
Reviewed-by: Jakub Czapiga <czapiga@google.com>
Commit-Queue: Jakub Czapiga <czapiga@google.com>