Data section on reset

[DL8]

I recently implemented a mechanism to place global data into D_MEM. This design will break on reset, because D_MEM doesn't survive reset. Furthermore, even if it does, globals may have changed at next reset, leading to unexpected results.

Two solutions I can think of:

1. Add "data ROM" that can be flashed and survives reset
2. Steal part of I_MEM for initial data. This baseline can possibly be copied into D_MEM on startup