Use `FLASH_PTR` macro to convert a flash address to a const pointer.
For real hardware it is just a cast, for emulator we subtract the
`FLASH_ORIGIN` and use it as index into the memory mapped flash file.
Make write access to flash with volatile pointers
Also use FLASH_PTR in DebugMemory* for now. This allows for reading and
writing the flash in the emulator or just crash it by reading outside
the flash...
