grub-efi: Use GRUB_IMAGE_PREFIX from base recipe

The main recipe (https://github.com/yoctoproject/poky/blob/master/meta/recipes-bsp/grub/grub-efi_2.12.bb#L62C46-L62C63) use GRUB_IMAGE_PREFIX in order to prefix the name of the final binary. Currently there is two binaries if the EFI_PROVIDER is grub-efi. This prefix shouldn't be used?

(I have patch for this issue, but for kirkstone branch. I may try to port it)