Kernel: Fix booting from "inactive" MBR partitions

Apparently you can boot from any MBR partition, not just the one labeled
as "bootable" or "active". The only ones you don't want to boot from are
the ones that don't exist.

Kernel/Devices/MBRPartitionTable.cpp

@@ -53,9 +53,17 @@ RetainPtr<DiskPartition> MBRPartitionTable::partition(unsigned index)
     kprintf("MBRPartitionTable::partition: status=%#x offset=%#x\n", entry.status, entry.offset);
 #endif
 
-    if (entry.status == 0x00) {
+    if (entry.offset == 0x00) {
+#ifdef MBR_DEBUG
+    kprintf("MBRPartitionTable::partition: missing partition requested index=%d\n", index);
+#endif
+
         return nullptr;
     }
 
+#ifdef MBR_DEBUG
+    kprintf("MBRPartitionTable::partition: found partition index=%d type=%x\n", index, entry.type);
+#endif
+
     return DiskPartition::create(m_device.copy_ref(), entry.offset);
 }