vgabios: Don't declare custom internal BDA storage in std/bda.h

The vgabios uses storage in the BDA at offset 0xb9 for internal custom
storage (the contents do not appear to be part of any bios standard).
Move the description of this custom vgabios area from std/bda.h to
vgasrc/vgabios.h.  Add two new macros (GET_BDA_EXT and SET_BDA_EXT).
This should make it more clear that the area is for custom internal
storage.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
4 files changed