Add instructions to copy the initram for signed kernel

We need an initram to boot the system, but the instructions for signing
the kernel did not include to copy an existing one for the signed
kernel. This commit adds these instructions. See discussion of #261 for
more info.

SIGNING.md

@@ -82,14 +82,20 @@ sudo mokutil --list-enrolled
 sudo sbsign --key MOK.priv --cert MOK.pem /boot/vmlinuz-[KERNEL-VERSION]-surface-linux-surface --output /boot/vmlinuz-[KERNEL-VERSION]-surface-linux-surface.signed
 ```
 
-8. Update your grub-config
+8. Copy the initram of the unsigned kernel, so we also have an initram for the signed one.
+```
+sudo cp /boot/initrd.img-[KERNEL-VERSION]-surface-linux-surface{,.signed}
+```
+
+9. Update your grub-config
 ```
 sudo update-grub
 ```
 
-9. Reboot your system and select signed kernel. If booting works, you can remove the unsigned kernel:
+10. Reboot your system and select the signed kernel. If booting works, you can remove the unsigned kernel:
 ```
 sudo mv /boot/vmlinuz-[KERNEL-VERSION]-surface-linux-surface{.signed,}
+sudo mv /boot/initrd.img-[KERNEL-VERSION]-surface-linux-surface{.signed,}
 sudo update-grub
 ```