.gitignore: Ignore build directories

.gitignore: Ignore build directories

This is what we're ignoring:

  $ tree
  .
  |-- builds            # compiled and seed stages
  |   `-- default
  |       |-- stage1-i686-2013.1.tar.bz2
  |       ...
  |       `-- stage3-i686-2013.1.tar.bz2.DIGESTS
  |-- iso               # compiled ISOs
  |   |-- swc-x86.iso
  |   |-- swc-x86.iso.CONTENTS
  |   `-- swc-x86.iso.DIGESTS
  |-- kerncache         # cached kernel compilation
  |   `-- default
  |       |-- livecd-stage1-i686-2013.1  # empty (no kernel in livecd-stage1)
  |       |-- livecd-stage2-i686-2013.1  # lots of kernel compilation stuff
  |       ...
  |-- packages          # binary packages from per-stage emerges
  |   `-- default
  |       |-- livecd-stage1-i686-2013.1  # emerged during livecd-stage1
  |       ...
  |       `-- stage3-i686-2013.1         # emerged during stage3
  ...
  |-- snapshot_cache    # unpacked portage snapshots
  |   `-- 20130208
  |-- snapshots         # packed portage snapshots
  |   |-- portage-20130208.tar.bz2
  |   |-- portage-20130208.tar.bz2.CONTENTS
  |   `-- portage-20130208.tar.bz2.DIGESTS
  ...
  `-- tmp               # unpacked stage chroots
      |-- default
      |   |-- livecd-stage1-i686-2013.1
      |   |-- livecd-stage2-i686-2013.1
      |   |-- stage1-i686-2013.1
      |   |-- stage2-i686-2013.1
      |   `-- stage3-i686-2013.1
      `-- portage
          `-- portage   # unpacked portage snapshot