shill: Generate intermediate build artifacts under a directory.

This makes the cros_workon_make build flow cleaner because all intermediate
files are stuffed under a build directory. The patch also removes the .gitignore
rule because its contents can be easily hardcoded now.

Note that with little effort (and some ebuild changes) we can make it possible
to cros_workon_make for different boards without the need for --scrub across
boards.

BUG=chromium-os:25434
TEST=unit tests, both with cros_workon_make and emerge-$board

Change-Id: I64cd23fb3ea2d250371ff218b43a66548552e7ff
Reviewed-on: https://gerrit.chromium.org/gerrit/16270
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c31583a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+*~
+/build/
+/shill
+/shill_unittest