(MESS) a7800.c:  [Fabio Priuli]
  - Fixed YM2151 clock in XM expansion, thanks to info from 
    tep392 and GroovyBee
  - Added support for homebrew boards MegaCart+ and
    VersaBoard by CPUWIZ (ROM respectively up to 512K
    and 256K + 32K of RAM)
  - Misc cleanups


out of whatsnew: I have also added, for debugging purposes, a VersaBoard with SG 9banks + POKEY at 0x0450
in the hope to find out exactly what goes currently wrong in the emulation of some homebrew games... let's see...