config.py: get_architecture() accepts base arch as optional first argument
command.py: allow shorthand specification of architecture for `build`

Using the changes in the previous commit, we can construct a build
architecture for the user provided a base architecture name.

...hey, it works for linux-x86_64, anyway... will probably need tweaks
for e.g. win32, bluegene{p,q}, etc.
command.py: build: default to "all" when --purge is given