make top-level .gitignore only apply to top-level files/directories (#1960)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@7728 3c298f89-4303-0410-b956-a3cf2f4a3e73

.gitignore

-.config*
-bin
-build_*
-toolchain_build_*
-staging_dir_*
-tmp
-dl
-tool_build
+/.config*
+/bin
+/build_*
+/toolchain_build_*
+/staging_dir_*
+/tmp
+/dl
+/tool_build