.gitignore: unify duplicated rules

A lot of subdirs duplicate .gitignore settings, so pull the standard
stuff out into the top level.

Also clean up the autotool stuff in p2p/ which isn't used anymore.

BUG=None
TEST=`git status` looks sane

Change-Id: I708d5ed91970bbf5a327b024ff6ebdb402782c3f
Reviewed-on: https://chromium-review.googlesource.com/212226
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/.gitignore b/.gitignore
index 9976a46..d6dd317 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,19 +1,5 @@
-#
-# NOTE! Please use 'git ls-files -i --exclude-standard'
-# command after changing this file, to see if there are
-# any tracked files which get ignored after the change.
-#
-
-# Standard excludes
-*~
-*.a
-*.o
-
 # Build/test generated files
-*.gcda
-*.gcno
 /*.pub.pem
-/.sconsign.dblite
 /app.info
 /delta_generator
 /html/
@@ -23,5 +9,3 @@
 /update_engine.dbusserver.h
 /update_engine_client
 /update_engine_unittests
-/update_metadata.pb.cc
-/update_metadata.pb.h