msx2: fix rom bootstrap

Hi Alan,
   the bootstrap from rom is now working, but vdpinit turns out it works only when booting from msx-dos because it wipes the font data from vram before actually moving it; so I have added a workaround.

   you will see the bootstrap needs to be in the bank 1 in order to be detected by the bios, but is possible to have code in the cartridge in lower addresses. I am using the BOOT segment for that. The problem is that then CODE is too big to fit in bank0 and overlaps the bootstrap, so I have moved some files to CODE2.

   handling the secondary slots is hell, so I have just taken the code from the bios itself.

cheers,
E.-