release: Support r/o /usr/ports for cloudware

Set WRKDIRPREFIX=/tmp/ports DISTDIR=/tmp/distfiles when building tools
needed for uploading cloudware images.

While I'm here, adjust the bsdec2-image-upload build target to match
the style used by the Azure/GCE/Vagrant Makefiles.

MFC after: 3 days

(cherry picked from commit 6c87aed39c3053cd338c1ec18fba5b1d773beca9)