add debug initramfs as seperate package

add debug initramfs as seperate package

"proxmox-backup-restore-image-debug", containing only the debug
initramfs, so depends on the base "proxmox-backup-restore-image" for the
kernel.

Adapt the init-shim to start an agetty on ttyS1, which the host
can use to connect to a root shell for debugging, and use
create_dir_all, since some debug packages seem to create /sys and /proc
as empty dirs already.

The build_initramfs.sh script is modified to include dependency
resolution via apt-rdepends, so debug packages like agetty (util-linux),
busybox and gdb can easily be added. This now builds both the regular
and the debug binary at once, to avoid downloading shared packages
twice.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>