Parameterize SCons file based on apio settings

[s-ol]

The SCons file is quite "magical" and undocumented at the moment. I needed to find it to understand these points for example:

• how is a PCF file located? which is chosen if there are multiple?
• how are testbench files recognized and treated?
• how are verilog source files recognized and treated?

IMO the second two should be documented in the docs for apio sim and apio build respectively. These informations are crucial for newcomers to start using apio, whether they have worked with FPGAs or icestorm before or not.

For solving the first issue, I would propose parameterizing the SCons file as follows:

• pcf - path to PCF file, default: glob *.pcf, error if multiple
• testbench - path to main testbench file, default: glob *_tb.v, error if multiple

these settings should be documented for and set in apio.ini. (btw: there is no documentation about apio.ini settings currently)

[roman3017]

• how are verilog source files recognized and treated?

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.