Kernel: Implement MBR partition loader (#168)

This implements a basic MBR partition loader, which removes the reliance
on a hard-coded filesystem offset in the stage2 init.

Kernel/Devices/MBRPartitionTable.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <AK/RetainPtr.h>
+#include <AK/Vector.h>
+#include <Kernel/Devices/DiskDevice.h>
+#include <Kernel/Devices/DiskPartition.h>
+
+#define MBR_SIGNATURE 0xaa55
+
+struct MBRPartitionEntry {
+    byte status;
+    byte chs1[3];
+    byte type;
+    byte chs2[3];
+    dword offset;
+    dword length;
+} __attribute__((packed));
+
+struct MBRPartitionHeader {
+    byte code1[218];
+    word ts_zero;
+    byte ts_drive, ts_seconds, ts_minutes, ts_hours;
+    byte code2[216];
+    dword disk_signature;
+    word disk_signature_zero;
+    MBRPartitionEntry entry[4];
+    word mbr_signature;
+} __attribute__((packed));
+
+class MBRPartitionTable {
+    AK_MAKE_ETERNAL
+
+public:
+    MBRPartitionTable(Retained<DiskDevice>&& device);
+    ~MBRPartitionTable();
+
+    bool initialize();
+    RetainPtr<DiskPartition> partition(unsigned index);
+
+private:
+    Retained<DiskDevice> m_device;
+
+    ByteBuffer read_header() const;
+    const MBRPartitionHeader& header() const;
+
+    byte m_cached_header[512];
+};