The `fastboot fetch` command is a wrapper around
the fetch protocol. It:
- getvar max-fetch-size
- get the size of the partition
- read the data by chunks, chunk size = max-fetch-size.
The name of the partition is passed directly to the device
(with the usual has-slot magic for flashing etc.) If we support
fetching partitions other than vendor_boot in the future, no change
in the driver is needed.
Bug: 173654501
Test: manual
Change-Id: Ie576eea668234df236b096a372e65c5e91c1e48c
bcd2770207