diskimage-builder usually provides defaults that work out of the box.
One default that does not work outside of x86 land is Ubuntu distro
mirror url. Considering there are only two valid default options, we can
automatically choose a better default.
This patch changes behavior only for architectures known to be using
http://ports.ubuntu.com/ubuntu-ports. All others still would use
http://archive.ubuntu.com/ubuntu as default. It provides some guarantee
that we do not introduce a regression.
Change-Id: If95a64bac0c88f30736da4bae7f1fdce126c0bf6
4d4417c9a6