linux: Add DEFAULT_PIVOT_ROOT var which can be changed at build time.
linux: Add DEFAULT_PIVOT_ROOT var which can be changed at build time.

This allows the default pivot root used by crosvm to be defined at
build time. If it is not set, /var/empty is used.

BUG=chromium:934513,chromium:933582
TEST=PreCQ passes

Change-Id: I2b461170e6b75324740d4ff709fd599085cefe18
Reviewed-on: https://chromium-review.googlesource.com/1483330
Commit-Ready: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm