Use uppercase UEFI file path on loong64

Olli Janatuinenrequested to merge
olljanat/debian-cd:loong64-efi-path into master

Loong64 machines are searching boot file from path "EFI/BOOT/BOOTLOONGARCH64.EFI" which matches to what is defined in UEFI spec.

Unfortunately ISO 9660 file system implementation in their Loongson firmware does not fully implement Joliet extension which why this path needs to be all caps.

There have been some discussion in mailing list about if correctly place to fix this is Debian installer or Loongson firmware but because only ISO 9660 file system looks to be affected my recommendation would be to include this change as it makes situation simpler for end users.

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