maixbit: add chip datasheet link and reformat code

src/machine/board_k210.go

@@ -1,5 +1,7 @@
 // +build maixbit
 
+// Chip datasheet: https://s3.cn-north-1.amazonaws.com.cn/dl.kendryte.com/documents/kendryte_datasheet_20181011163248_en.pdf
+
 package machine
 
 // K210 IO pins.

src/runtime/runtime_k210.go

@@ -24,7 +24,9 @@ func main() {
 	initPLIC()
 
 	// Only use one hart for the moment.
-	if riscv.MHARTID.Get() == 0 {
+	if riscv.MHARTID.Get() != 0 {
+		abort()
+	}
 
 	// Reset all interrupt source priorities to zero.
 	for i := 0; i < kendryte.IRQ_max; i++ {
@@ -47,9 +49,6 @@ func main() {
 	initPeripherals()
 	run()
 	abort()
-	} else {
-		abort()
-	}
 }
 
 func initPLIC() {