docs: riscv: convert boot-image-header.txt to ReST

Convert this small file to ReST format by:
- Using a proper markup for the document title;
- marking a code block as such;
- use tags for Author and date;
- use tables for bit map fields.

While here, fix a broken reference for a document with is
planned but is not here yet.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Reviewed-by: Atish Patra <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/riscv/boot-image-header.txt Documentation/riscv/boot-image-header.rst

···
-				Boot image header in RISC-V Linux
-			=============================================
+=================================
+Boot image header in RISC-V Linux
+=================================
 
-Author: Atish Patra <[e-mail]>
-Date  : 20 May 2019
+:Author: Atish Patra <[e-mail]>
+:Date:   20 May 2019
 
 This document only describes the boot image header details for RISC-V Linux.
-The complete booting guide will be available at Documentation/riscv/booting.txt.
 
-The following 64-byte header is present in decompressed Linux kernel image.
+TODO:
+  Write a complete booting guide.
+
+The following 64-byte header is present in decompressed Linux kernel image::
 
 	u32 code0;		  /* Executable code */
-	u32 code1; 		  /* Executable code */
+	u32 code1;		  /* Executable code */
 	u64 text_offset;	  /* Image load offset, little endian */
 	u64 image_size;		  /* Effective Image size, little endian */
 	u64 flags;		  /* kernel flags, little endian */
 	u32 version;		  /* Version of this header */
-	u32 res1  = 0;		  /* Reserved */
-	u64 res2  = 0;    	  /* Reserved */
+	u32 res1 = 0;		  /* Reserved */
+	u64 res2 = 0;		  /* Reserved */
 	u64 magic = 0x5643534952; /* Magic number, little endian, "RISCV" */
 	u32 res3;		  /* Reserved for additional RISC-V specific header */
 	u32 res4;		  /* Reserved for PE COFF offset */
···
 ARM64 header. Thus, both ARM64 & RISC-V header can be combined into one common
 header in future.
 
-Notes:
+Notes
+=====
+
 - This header can also be reused to support EFI stub for RISC-V in future. EFI
   specification needs PE/COFF image header in the beginning of the kernel image
   in order to load it as an EFI application. In order to support EFI stub,
   code0 should be replaced with "MZ" magic string and res5(at offset 0x3c) should
   point to the rest of the PE/COFF header.
 
-- version field indicate header version number.
-	Bits 0:15  - Minor version
-	Bits 16:31 - Major version
+- version field indicate header version number
+
+	==========  =============
+	Bits 0:15   Minor version
+	Bits 16:31  Major version
+	==========  =============
 
   This preserves compatibility across newer and older version of the header.
   The current version is defined as 0.1.
···
   extension for RISC-V in future. For current version, it is set to be zero.
 
 - In current header, the flag field has only one field.
-	Bit 0: Kernel endianness. 1 if BE, 0 if LE.
+
+	=====  ====================================
+	Bit 0  Kernel endianness. 1 if BE, 0 if LE.
+	=====  ====================================
 
 - Image size is mandatory for boot loader to load kernel image. Booting will
   fail otherwise.

Documentation/riscv/index.rst

···
 .. toctree::
     :maxdepth: 1
 
+    boot-image-header
     pmu
 
 .. only::  subproject and html