Add leading / to prefix of network boot image for d-i

Add leading / to prefix of network boot image for d-i

The win32-loader package relies on the hosted netboot debian-installer
files.

The grubx64.efi file is located at
https://deb.deb.debian.org/debian/dists/stable/main/installer-amd64/current/images/netboot/gtk/debian-installer/amd64/grubx64.efi
and originates from the grub2 source package.

This grubx64.efi file includes the preconfigured prefix:
($root)debian-installer/amd64/grub.

This prefix lacks the leading / in front of debian-installer. Therefore
no grub.cfg file located on a local storage device can be accessed via
this preconfigured prefix.

The normal network boot image uses ($root)/grub as its prefix. As a
stopgap the win32-loader package (experimental) transforms the above
mentioned grubx64.efi file into a normal network boot image (also known
as grubnetx64.efi.signed of the grub-efi-amd64-signed_1+2.02+dfsg1+20
package). As a consequence the grub.cfg file could be accessed at the
($root)/grub prefix.