boot/grub2/readme.txt: don't specify /dev/loop0

losetup -f returns the next free loop device, which may not be
/dev/loop0.  If you blindly follow the readmy you may end up destroying
an existing device.

Make it more robust with a variable to store the loop device.

Signed-off-by: Cherniaev Andrei <dungeonlords789@naver.com>
[Arnout: keep the actual losetup atomic]
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit 7dd56b6cd928ee36c96a697075e3fa47b0c52d5e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/grub2/readme.txt

@@ -10,20 +10,20 @@ Notes on using Grub2 for BIOS-based platforms
       is enough free space *before* the first partition to
       store Grub2. Leaving 1 MB of free space is safe.
 3. Setup loop device and loop partitions
-   sudo losetup -f disk.img
-   sudo partx -a /dev/loop0
+   loop_dev=$(sudo losetup -f disk.img)
+   sudo partx -a "$loop_dev"
 4. Prepare the root partition
-   sudo mkfs.ext3 -L root /dev/loop0p1
-   sudo mount /dev/loop0p1 /mnt
+   sudo mkfs.ext3 -L root "${loop_dev}p1"
+   sudo mount "${loop_dev}p1" /mnt
    sudo tar -C /mnt -xf output/images/rootfs.tar
    sudo umount /mnt
 5. Install Grub2
    sudo ./output/host/sbin/grub-bios-setup \
         -b ./output/host/lib/grub/i386-pc/boot.img \
-        -c ./output/images/grub.img -d . /dev/loop0
+        -c ./output/images/grub.img -d . "$loop_dev"
 6. Cleanup loop device
-   sudo partx -d /dev/loop0
-   sudo losetup -d /dev/loop0
+   sudo partx -d "$loop_dev"
+   sudo losetup -d "$loop_dev"
 7. Your disk.img is ready!
 
 Using genimage
@@ -65,22 +65,22 @@ Notes on using Grub2 for x86/x86_64 EFI-based platforms
     - Create a second partition, type 8300, for the root
       filesystem.
 3. Setup loop device and loop partitions
-   sudo losetup -f disk.img
-   sudo partx -a /dev/loop0
+   loop_dev=$(sudo losetup -f disk.img)
+   sudo partx -a "$loop_dev"
 4. Prepare the boot partition
-   sudo mkfs.vfat -n boot /dev/loop0p1
-   sudo mount /dev/loop0p1 /mnt
+   sudo mkfs.vfat -n boot "${loop_dev}p1"
+   sudo mount "${loop_dev}p1" /mnt
    sudo cp -a output/images/efi-part/* /mnt/
    sudo cp output/images/bzImage /mnt/
    sudo umount /mnt
 5. Prepare the root partition
-   sudo mkfs.ext3 -L root /dev/loop0p2
-   sudo mount /dev/loop0p2 /mnt
+   sudo mkfs.ext3 -L root "${loop_dev}p2"
+   sudo mount "${loop_dev}p2" /mnt
    sudo tar -C /mnt -xf output/images/rootfs.tar
    sudo umount /mnt
 6  Cleanup loop device
-   sudo partx -d /dev/loop0
-   sudo losetup -d /dev/loop0
+   sudo partx -d "$loop_dev"
+   sudo losetup -d "$loop_dev"
 7. Your disk.img is ready!
 
 To test your i386/x86-64 EFI image in Qemu