Clear LP/RTC RAM

[bjoernQ]

This clears LP/RTC RAM before loading ULP/LP code to make sure .bss is zeroed.

It could also be done in the runtime of the ULP/LP code but that unnecessarily eats a few bytes from the already scarce RAM.
We could also only clear the memory dedicated to .bss but clearing the whole 8/16k shouldn't make a big difference. If it ever does we can revise this decision.

Additionally while on it this renames the ulp-riscv-hal to esp-ulp-riscv-hal

Fixes #859

[MabezDev]

What happens if the main CPU goes into deep sleep and wakes up? Won't this wipe an already running LP/ULP application if it's running?

[bjoernQ]

I think what we should do after deep sleep is checking the reset cause - if it was a cold start the application would load a new binary otherwise let the LP core continue. I admit this case isn't totally thought through or even tested.

But even without this - without any further checks - we would load the binary again into LP ram and overwrite .data. So I guess some care is always needed for this case

[MabezDev]

Good point, we can solve that problem when we get to it :)