fastboot: add preflash command to help with large downloads+flash

Without the partition name, the bootloader would have to store all
of the data until the next flash command, which potentially wastes
time.
To help the bootloader deal with the data more efficiently, we now
issue a flashing hint ("preflash:%s", partname).
"preflash:" is used to indicate that the following download
should ignore any previous "preflash:%s".

Bug: 6045918
Change-Id: I4e9708f64f28a4781bde14eb3714d36b281ec5be