Don't install Recommends during base system installation

Analysis has shown that the previous change to install Recommends as set
in debconf leads to inconsistent results (as debootstrap does not honor
Recommends) and would require --no-recommends for most system boot related
packages (udev, initramfs-tools, mdadm, etc.) to ensure consistency between
installs and to avoid premature installation of e.g. exim4.

Therefore initially set up APT not to install Recommends and reconfigure it
at the very end according to the debconf setting.

References:
- http://lists.debian.org/debian-boot/2010/02/msg00033.html

r62344