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>
4 files changed