build:next: fix ext_flash_release target

The intent was that ext_flash_target build a tarball with all the bits
needed to flash a Nexus board. When running on Renode the builtins
are built but not packaged as they are loaded directly into memory. For
Nexus we need to package the builtins so they can be written to flash
from where they are loaded by tock.

Change-Id: I5d47b1ab9b8a58f18edd42b0930764552bca5d12