Building stage1 only

[Ömer Sinan Ağacan]

(I know I asked this many times in IRC channel but I don't remember getting any
answers. I apologize if anyone had answered this on IRC channel and I missed)

With current build system, even if I choose "devel1" it always builds stage2
compiler too. A comment in build.mk says that it's for working on stage1
compiler, so I'd expect it to build only stage1.

The reason I want stage1-only builds is the time it takes to build both stages.
It takes approximately 30 minutes and if I could reduce this down to half that
would greatly improve my work flow.

Most of the time I don't need a full build, but in my experience, it's very
easy to end up in a state where `make` simply can't build, and only
easy fix is a
`make clean`, which costs 30 minutes because then I have to build from scratch.
This happens way too often, and so I need a stage1-only build.

Thanks..