Feature request: 48K memory support

[rhester72]

A few games, such as Beach Head by Access Software, will not run unless precisely 48K of memory is detected

[thesnable]

Second this

[woj76]

This is fixed in https://github.com/woj76/Atari800_MiSTer/tree/pbi_bios, I am preparing a collective pull request from that branch. The new options in the core include pure Atari 800 mode with 16K, 32K, 48K, and 52K, as well as 4MB Axlon type extension. Non-existent memory handing (floating bus) and cold power on memory initialization have also been fixed to remedy all kinds of quirks the 800 OS and some of the software have.