Allow to control the order of the source files.

[zapta]

Moved from #176 (comment)

"I also find this request useful. It would be also nice if there is a way to specify the source files order. The current order seems to be alphabetical, which may not work in some cases. An example is building picosoc. One needs to list picosoc.v before picorv32.v so one cannot use apio build as is. The current workarounds are using Makefile or using apio raw or modifying SConstruct script or renaming the original files. Maybe having a possibility to have an ordered list of all sources in a config file could be beneficial. If the ordered list of sources is missing then existing alphabetical order of all sources could work."