BACKPORT: CHROMIUM: add DEVTMPFS_SAFE support

    If DEVTMPFS_SAFE==y, automount devtmpfs with nosuid and noexec.

    BUG=chromium-os:32629
    TEST=adhoc
    mount | grep devtmpfs

    Change-Id: I7a330fb7bd1e0da7e42fe2194a958c6878c0ce68
    Signed-off-by: Elizabeth Fong-Jones <ellyjones@chromium.org>
    Reviewed-on: https://gerrit.chromium.org/gerrit/38061
    Reviewed-by: Olof Johansson <olofj@chromium.org>
Add DEVTMPFS_SAFE to the defconfig