flashy-5b1099b

yosemite4: flashy: add flash procedure

Summary: Copy flash procedure from bletchley for Yv4 update.

Test Plan: Build flashy.

Reviewed By: kawmarco

Differential Revision: D52739771

fbshipit-source-id: 801c68f1b7c3d5eeb5aa2450bc5c2f0e3aea3826