install_toolchain: work better with empty root

For testing purposes, it's nice to be able to run this script with an
empty dir.  Add a few more sanity checks to make that happen.

BUG=None
TEST=`rm -rf foo &&
      mkdir foo &&
      ./install_toolchain --board daisy --board_root $PWD/foo` works

Change-Id: Ic3fb293d65faa02fb824866c5a7f3e0e9e834093
Reviewed-on: https://chromium-review.googlesource.com/171621
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed