build initramfs: download packages into per-flavor dirs

build initramfs: download packages into per-flavor dirs

commit f773602bd35c84c292649fb1c6e215050f25c21e introduced pre-caching
packages in the source-package, but resulted in both initrds getting
all packages installed:
* DOWNLOAD_ONLY=1 .../build_intiramfs.sh puts all packages in to pkgs
* installation when running w/o DOWNLOAD_ONLY installs all files
  matching '*.deb' - which results in all packages being installed

Additionally the TO_DOWNLOAD variable contained all packages anyways
(apt-rdepends yields packagenames - the file-existance check matches
on deb-filenames (containing version+arch) - so this was stripped out

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>