[emac][board] Change the board name of emac to be vim3

Otherwise assembly tool will complain the hw_revision does not match
that of partition config

Change-Id: I431119ba82f33724bc5a7725d5620af049d1a4e7
Reviewed-on: https://fuchsia-review.googlesource.com/c/boards/emac/+/1112212
Reviewed-by: Aidan Wolter <awolter@google.com>
Commit-Queue: Jiaming Li <lijiaming@google.com>