Machine config usability enhancements for testing

[edubart]

Context

There are some inflexibilities in the Lua configuration that makes more boilerplate for us to create tests. Like the impossibility of creating a machine with an empty rom (we have to do tricks to create empty files), and the requirement of always needing to get flash drive length (this could be auto detected).

Definition of Done

• Support creating a machine without a ROM image filename. ROM could still be overwritten with a memory write.
• Allow omitting length from flash drive configs, instead fill this automatically by getting the file size.

[diegonehab]

Isn't this merged already, @edubart ?

[edubart]

No, it's contained in #77 , which is not merged yet because it needs more approvals.