buildsys: fix dependencies for bootstrap files

buildsys: fix dependencies for bootstrap files

avoid removing and unzipping the bootstrap source archive as many
times as ${BTDATA} has file entries, add an intermediate target for
the directory, which is the producer for all those BTDATA files, and
that directory then depends on the zip archive.

I mean, it would be even better to just use the libjs-bootstrap
package (and jQuery for that matter) but that a little bigger change
for now

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>