fastboot: Add 'get_staged' command

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

New user-level command usage:

 * fastboot get_staged <outfile>
   Reads staged data from the last command handled by the device. If the
   last command did not result in staged data, this command will fail.

This enables data staged by OEM commands to be transferred from device
to host. get_staged wraps new device command "upload". Fastboot
clients are not required to support "upload", so get_staged won't
work on all devices.

Bug: 36002804
Test: Implemented "upload" in fastboot on imx6ul. Verified that uploading
      ~100K data from the device works.
Change-Id: I5b1a1ce023f362062505ee62746ea8ab6f36bfbf
(cherry-picked from commit 83a875de994bf48f0faa2a8a23ceb0b8f52b6b04)