shill: remove static linking to libbootstat

libbootstat has been merged into libchromeos-core.
This removes it from the libraries.

BUG=chromium:387186
TEST=FEATURES=test emerge-amd64-generic platform2
TEST=trybot run on daisy, link, duck, mario and lumpy-incremental.

CQ-DEPEND=CL:205782

Change-Id: If01cd46f2889c2d92d559e0f77d8a68a304911ac
Reviewed-on: https://chromium-review.googlesource.com/205767
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>