UEFI Unified Kernel Image for Debian Installer on riscv64

On the riscv64 port, the default boot method is UEFI, with U-Boot typically used as the firmware. This approach aligns more closely with other architectures, which avoid developping riscv64 specific code. For advanced users, booting using U-Boot and extlinux is possible, thanks to the kernel being built with CONFIG_EFI_STUB=y . The same applies to the Debian Installer, which is provided as ISO…