[pcbios] Add additional sanity check for bogus e820 map

At POST time some BIOSes return invalid e820 maps even though
they indicate that the data is valid.  We add a check that the first
region returned by e820 is RAM type and declare the map to be invalid
if it is not.

This extends the sanity checks from 8b20e5d ("[pcbios] Sanity-check
the INT15,e820 and INT15,e801 memory maps").

src/arch/i386/firmware/pcbios/memmap.c

@@ -201,6 +201,13 @@ static int meme820 ( struct memory_map *memmap ) {
 			break;
 		}
 
+		/* If first region is not RAM, assume map is invalid */
+		if ( ( memmap->count == 0 ) &&
+		     ( e820buf.type != E820_TYPE_RAM ) ) {
+		       DBG ( "INT 15,e820 failed, first entry not RAM\n" );
+		       return -EINVAL;
+		}
+
 		DBG ( "INT 15,e820 region [%llx,%llx) type %d",
 		      e820buf.start, ( e820buf.start + e820buf.len ),
 		      ( int ) e820buf.type );