[mbr] add LBA sector readout, as required by some BIOSes

* I'm looking at you, Dell Optiplex!
* also fix boot in case no secondary bootable drive exists
* also adds a readme

res/mbr/readme.txt

@@ -0,0 +1,49 @@
+Rufus: The Reliable USB Formatting Utility - Custom MBR
+
+# Description
+
+This directory contains all the resources required to create an MBR that prompts
+the user for boot selection, when a second bootable device (typically bootable
+fixed HDD) is reported by the BIOS.
+
+This aims at mimicking the Microsoft Windows optical installation media feature,
+which may be necessary on for WinPE 2.x or earlier based installations.
+
+# Compilation
+
+Any gcc suite (except possibly the X-Code one on OS-X) should be able to compile
+the MBR by invoking 'make'. A 'make dis', that produces a disassembly dump is
+also provided for your convenience.
+
+# Primer
+
+The way this bootloader achieves the feature highlighted above is as follows:
+1. An attempt to read the MBR of the second bootable drive (0x81) is made
+   through INT_13h (in either CHS or LBA mode depending on the extensions 
+   detected)
+2. If that attempts succeeds, then the partition table from the newly read MBR
+   is checked for an active/bootable entry.
+3. If such a partition is found, a prompt is displayed to the user and an RTC 
+   timer interrupt (INT_8h) override is added so that dots are displayed at
+   regular interval. Then the keyboard is checked for entry.
+4. If the user presses a key, the first partition boot record from the USB is
+   read (according to the values found in the USB MBR partition table) and
+   executed
+5. If no key is pressed, then an INT_13h (disk access interrupt) override is
+   added to masquerade the second bootable drive (0x81) as the first one (0x80)
+   so that the Windows second stage installer, or any other program relying on
+   BIOS disk access, behave as if there was no USB drive inserted.
+6. In case there was a failure to read the second bootable drive's MBR, or no
+   active partition was detected there, the USB is booted without prompts.
+
+# Limitations
+
+* If you are using software RAID or a non-conventional setup, the second
+  bootable disk may not be accessible through the BIOS and therefore the USB
+  will always be booted
+* If the bootable HDD uses LILO, a "LILO - Keytable read/checksum error" will
+  be displayed when trying to boot it.
+* This MBR currently does not masquerade the bootable USB drive as secondary
+  (0x81) therefore an installation program ran from USB to install an OS on
+  an HDD may still configure that disk as the second drive, and prevent it to
+  properly boot later on.