Make possible to install for i386 target from same ISO image

[serhepopovych]

It is supported to add multiple architectures to ISO image with simple-cdd. Currently only amd64 supported. Reprepro is able to create mirrors for multiple architectures too.

Adding i386 can be useful for installations on old hardware or new embedded one that has only
32-bit CPU instructions.

For this feature to work:

1. Use Docker to build debian-installer and implement build system Makefiles in Debian-installer for specific Debian release builds using Docker. #2
2. Build debian-installer for amd64 and i386.
3. Enhance isolinux/grub to detect CPU instructions and select appropriate kernel/initrd.
   Note that debian-installer starting from wheezy uses separate directories (install.386 and
   install.amd) for i386 and amd64 kernel and initrd imgages.

[serhepopovych]

Now it should be possible to build either i386 or amd64 ISO images only. Multiarch is tracked by this issue.

[serhepopovych]

Not critical for first-release. Postponed.