fastboot: Add 'stage' command

(cherry-picked from internal nyc-iot-dev to AOSP)

New user-level command usage:

 * fastboot stage <infile>
   Sends the contents of <infile> to the device to stage for use in the
   next command.

This enables OEM commands to use data downloaded from host to device.

Bug: 35811075
Test: Manual test on imx6ul
Change-Id: I483a18f9f4205d3289ee524656b9d741b16e9fe6
(cherry-picked from commit 001c75c6c0fe6a70a1db2a65253ab3c43ec17d46)