[SETUPLIB] Code re-organization in bootsup.c and fsutil.c.

- Move the actual VBR bootcode installation helpers into fsutil.c
  (they depend on the selected filesystem).

- Introduce InstallBootCodeToDisk() and InstallBootCodeToFile()
  and bootcode.c helpers, in order to replace the several functions
  that were duplicating the same code.

base/setup/lib/bootcode.h

@@ -0,0 +1,37 @@
+/*
+ * PROJECT:     ReactOS Setup Library
+ * LICENSE:     GPL-2.0+ (https://spdx.org/licenses/GPL-2.0+)
+ * PURPOSE:     BootCode support functions.
+ * COPYRIGHT:   Copyright 2020 Hermes Belusca-Maito
+ */
+
+#pragma once
+
+#ifdef SECTORSIZE
+#undef SECTORSIZE
+#endif
+#define SECTORSIZE 512
+
+typedef struct _BOOTCODE
+{
+    PVOID BootCode;
+    ULONG Length;
+} BOOTCODE, *PBOOTCODE;
+
+NTSTATUS
+ReadBootCodeByHandle(
+    IN OUT PBOOTCODE BootCodeInfo,
+    IN HANDLE FileHandle,
+    IN ULONG Length OPTIONAL);
+
+NTSTATUS
+ReadBootCodeFromFile(
+    IN OUT PBOOTCODE BootCodeInfo,
+    IN PUNICODE_STRING FilePath,
+    IN ULONG Length OPTIONAL);
+
+VOID
+FreeBootCode(
+    IN OUT PBOOTCODE BootCodeInfo);
+
+/* EOF */