[PATCH] Reimplementing the legacy "map" command

[Javier Martín]

Hi again everybody. After some hacking with the GRUB internals, I've
completed a reimplementation of the legacy "map" command that allowed
the BIOS drives to have their numbers reassigned, in order to boot OSes
whose boot code requires their installation to be located in the first
HD (BIOS drive 0x80).

This implementation gets next to the status of the command in GRUB
Legacy, with the main changes being:

* Command: now called "drivemap", attending to suggestions in this list
* UI: the less complete part of the job. The syntax is now as follows:
        drivemap (hd1) 0x80 # Makes (hd1) appear as the first BIOS drive
      i.e. the second argument is now any BIOS drive number instead
      of another device. This change is in part due to my lack of time,
      but also because others here pointed that it would be useful to
      allow arbitrary mappings. If required, old syntax could be
      reimplemented in short time.
* Backend: the command is now a module, but there is also code in the
           several files like the machine-specific loader.S, the loader
           header file and the makefiles.
* Backend: a preboot hook system has been added to the loader backend,
           allowing modules to register functions to be run just before
           the grub_X_real_boot function is called. The drivemap module
           uses this new functionality to get its install_int13_handler
           function to be called before booting the target OS without
           creating chaos in GRUB itself.
* Functionality: the new module is independent of chainloader, so its
                 functions can be applied with any target loader. What
                 effects will this have, I don't know.

This patch is just a preliminary version that has been tested only in
QEMU and Bochs, not on real hardware because I'm cut off from x86
computers from the time being (I'm running on a PPC mac now). I will be
back to town (and to my olde amd64 box) this weekend, but I wanted to
submit this so others could scrutinize it for obvious mistakes, and
maybe test it.

I've tried to respect the style rules I've seen, but please note that
this is my first patch. Cheers!

Habbit

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