Allow to set platform and architecture manually in ./do + update packaging

This allows overriding PLATFORM and MARCH variables in ./do by manually exporting them from the shell.

The packaging is updated to take advantage of this. This fixes builds in cross-architecture chroots (e.g. on i386 build servers running on amd64 kernels) on Debian.

This is also lays the groundwork for supporting cross-architecture builds in the future.