Add new machine GRiD GRiDcase 1520

AT/286 based early (1988) laptop with indestructible magnesium case.

Also add emulation of 3 specific Conner HDDs as required by stock 1520 BIOS.
It only works with these particular drives and requires them to be somewhat
slow or reads will hang. The alternative is to use RomBuster to patch your
BIOS.

The machine had 3 display options (640x480 plasma or 2 different LCDs).
For now we need to use stock CGA with mono amber monitor to get some of
the plasma feel. Next step is to add Yamaha V6366 video card support.

src/include/86box/machine.h

@@ -848,6 +848,9 @@ extern int machine_at_vpc2007_init(const machine_t *);
 /* m_at_t3100e.c */
 extern int machine_at_t3100e_init(const machine_t *);
 
+/* m_at_grid.c */
+extern int machine_at_grid1520_init(const machine_t *);
+
 /* m_elt.c */
 extern int machine_elt_init(const machine_t *);