Add stage=1 in mk/build.mk

[dfordivam]

With Stage1Only flag stage2 is still built...

# You may wonder why Stage1Only=YES still builds compiler/stage2. Quoting # simonmar in #7639:
#
#     "strictly speaking building compiler/stage2 is correct, because it is
#     the ghc package that can be used with [ghc-stage1]. If you don't
#     want to build it, then setting stage=1 in your mk/build.mk should
#     disable it."

[dfordivam]

or perhaps add this comment somewhere, so that while hacking one doesn't have to build stage2.

[dfordivam]

Will have to add an option here pkgs/development/compilers/ghc/head.nix