Reapply "release.sh: Add -jN to make release"

With the latest changes to release/Makefile, it is now possible to
run make release -jN without the build failing (at least in my
latest tests).

This reverts commit 7b707e797b2cd6265ba8f6215e59445e9efb9e97.

MFC after: 1 week

(cherry picked from commit 2c01e189ff190cf1618dedd01f196bca68298c44)