[Ubuntu] Enabling other architectures

[Sebastiaan Couwenberg]

Currently the UbuntuGIS packages are only built for amd64 & i386, but
Launchpad can also build the packages for arm64, armel, armhf, powerpc,
ppc64el & s390x.

There have been questions about packages for armhf by Rasberry Pi users,
since the Pi 2 and later should support that instead of the ARMv6
architecture variant specific to rasbian.

I think we should consider at enabling the armhf architecture since
there's been explicit demand.

Based on the experiences in Debian I recommended again enabling armel,
which not all packages support. It's a bit of a problematic architecture
and likely to no longer be a release architecture in Debian after stretch.

There has been no explicit demand for arm64, powerpc, ppc64el & s390x
packages, so I think we can leave those until demand is voiced.

With the debug packages enabled storage usage of the PPA will increase,
as will enabling more architectures, so we may need to requests
additional storage if we choose to enable armhf.

Thoughts, concerns, etc?

Kind Regards,

Bas

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