Figure out correct sbrkIndex when initializing the memory

[tomusdrw]

Related: #159

#215 makes the memory fixed-size, i.e. there is no way for the PVM program to allocate more memory via sbrk instruction.
This is needed since the JAM Service example we have is listing the writeable pages in initial-page-map, however typeberry requires the entire space between sbrkIndex and HeapEnd to not have any initial pages.

This is probably going to be resolved on it's own with SPI support.

[tomusdrw]

Fixed in #244