[build] Allow PIN_TOOL to be overridden.
[build] Allow PIN_TOOL to be overridden.

mit-carbon/Graphite-Cycle-Level,mit-carbon/Graphite,fhijaz/Graphite,victorisildur/Graphite,victorisildur/Graphite,nkawahara/Graphite,victorisildur/Graphite,8l/Graphite,mit-carbon/Graphite-Cycle-Level,fhijaz/Graphite,nkawahara/Graphite,fhijaz/Graphite,8l/Graphite,mit-carbon/Graphite,mit-carbon/Graphite-Cycle-Level,mit-carbon/Graphite-Cycle-Level,fhijaz/Graphite,nkawahara/Graphite,mit-carbon/Graphite,victorisildur/Graphite,nkawahara/Graphite,mit-carbon/Graphite,8l/Graphite,8l/Graphite