minijail: Add support for entering an existing VFS namespace.

Also, fix the Makefile while in there.

BUG=chromium:376987
TEST=security_Minijail0
CQ-DEPEND=CL:209242

Change-Id: I18877211549500cbb720805a2480b1cb3244c1e9
Reviewed-on: https://chromium-review.googlesource.com/209240
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/Makefile b/Makefile
index 204e64b..d931df0 100644
--- a/Makefile
+++ b/Makefile
@@ -81,5 +81,5 @@
 	@rm -f libminijail.so
 	@rm -f libminijail_unittest
 	@rm -f libsyscalls.gen.o libsyscalls.gen.c
-	@rm -f syscall_filter.o signal.o bpf.o util.o
+	@rm -f syscall_filter.o signal.o bpf.o util.o elfparse.o
 	@rm -f syscall_filter_unittest syscall_filter_unittest.o