Improvements to porting page

[Ivan-Velickovic]

Based of feedback from the mailing list.

• [ ] Include a link for getting development environment setup.
• [ ] Include step by step example for how to do the port, maybe taking a recently added port as an example and walking through the diff. For this it would be worth doing an ARM board port since RISC-V is almost the exact same process with fewer changes need (things like timer/serial driver go through SBI so they're not necessary to port on RISC-V platforms as one example).
• [ ] There should be instructions on how to build a standalone kernel so users can check that the kernel builds before trying to deal with sel4test.
• [ ] Maybe a link to Microkit's porting section in the manual.
• [ ] Is it worth mentioning at least that x86 platforms should 'just work' in case someone is wondering about that?

These are things I can think of right now, I will add to this list (or others can) if more comes to mind.

[Indanz]

The confusing bit is how Elfloader, sel4test and sel4test and the user libraries all depend on other stuff like the kernel and each other. E.g. the UART you need to get working for Elfloader, kernel and user space separately.

I don't think building the kernel separately will help in any way, that will only add more instead of less complexity. seL4test depends on files generated by the kernel build, so the kernel is build first anyway.