Fix missing unicode font for unsigned grub

In commit a4dc8c0f, the default font was changed to unicode. It worked for signed grub efi images, which is a "monolithic" grub that embeds everything, including the unicode font.

However for derivatives that don't use the signed grub from Debian, what we get is a "traditional" grub (ie. not monolithic), so the unicode font must be installed at the right location. And that's not the case, all we have is grub/font.pf2, while what we need is grub/fonts/unicode.pf2.

This commit fixes it. However, as a side-effect, it also breaks debian-cd, which expects to find file(s) grub/*.pf2, and fails if there's no such:

build/debian-cd stdout:   Adding EFI boot code for amd64 on CD1
build/debian-cd stderr: cp: cannot stat 'boot1/isolinux/grub-amd64/*.pf2': No such file or directory
build/debian-cd stdout:       FAILED: error 1
build/debian-cd stderr:     Failed to start disc 1, error 256
build/debian-cd stderr: make: *** [Makefile:487: image-trees] Error 1

Therefore a fix in debian-cd must also be applied.

Closes: #1038862