release: Publish non-uploaded cloudware bits

While most "cloudware" (AWS, Azure, GCP, Vagrant) images get uploaded
to their respective clouds, we have two images -- BASIC-CI and
BASIC-CLOUDINIT -- which are published via the FTP mirrors. We have
been handling these using a manual and error-prone process of copying
them from the internal build directories into the FTP staging tree,
but they should be handled using the "make install ftp-stage"
mechanism which is used for all the other published images.

This commit 1. Adds a new 'make cloud-install' target which is run as
part of 'make install' and installs BASIC-CI and BASIC-CLOUDINIT images,
and 2. Extends 'make ftp-stage' in Makefile.mirrors to copy those bits
into the FTP staging area.

As a side benefit, this will also make BASIC-CI and BASIC-CLOUDINIT
images available for arm64; until now they have only been available
for amd64.

MFC after: 1 week