Add instructions for modifying the HWID.

scripts/newbitmaps/HWID.txt

Index: scripts/newbitmaps/HWID.txt
diff --git a/scripts/newbitmaps/HWID.txt b/scripts/newbitmaps/HWID.txt
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+
+This describes how to change the HWID string that's displayed using the
+new-style BIOS bitmaps. For a more detailed explanation of what's going on,
+refer to the README file.
+
+The HWID is encoded in the GBB in two ways: An ASCII string that userspace
+can read from /sys/devices/platform/chromeos_acpi/HWID, and as a rendered
+bitmap that shows up on the BIOS screens.
+
+
+Here's how to generate a new bitmap blob with a modified HWID
+
+1. OUTSIDE the chroot, cd to
+   src/platform/vboot_reference/scripts/newbitmaps/strings
+
+2. Edit the file 'hwid.txt' to contain your new string.
+
+3. Run "./text_to_bmp hwid.txt" to create "hwid.bmp" which contains ONLY the
+   picture of the HWID string.
+
+4. Copy the hwid.bmp image to the directory where the rest of the bitmap
+   blob comes from (which might not be public yet). If you don't know where
+   that is, just use the example directory:
+
+     src/platform/vboot_reference/scripts/newbitmaps/images/1280x800
+
+5. cd to the bitmap source directory, INSIDE the chroot.
+
+6. Edit the .yaml file that describes the bitmap blob (if you're in the
+   1280x800 directory, it's called "unknown.yaml"). Change the line that
+   starts with "hwid:" to refer to your new hwid.bmp file (or replace the
+   existing hwid bitmap file with your new one).
+
+7. Run "bmpblock_utility -c unknown.yaml new_bitmaps.bin", to create the new
+   bitmap blob.
+
+8. Install the new_bitmaps.bin on your device (see below).
+
+
+
+
+The gbb_utility is used to replace both the ASCII string and the bitmap blob
+in a BIOS image. The typical proces is to do this as root on the target
+system. This will only work if THE BIOS write-protect is disabled.
+
+
+A. Read the existing BIOS from ROM (so you don't lose the VPD):
+
+    cd /mnt/stateful_partition
+    flashrom -r bios.bin
+
+B. Modify the GBB in the BIOS image to contain the new bits:
+
+    gbb_utility -s 'NEW HWID STRING 1234' -b new_bitmaps.bin bios.bin
+
+C. Write the modified BIOS back to ROM:
+
+    flashrom -w bios.bin