Add raw-efi test + documentation

Signed-off-by: Chris Irrgang <chris.irrgang@gmx.de>

test/cases/010_platforms/000_qemu/060_run_raw_efi/test.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+# SUMMARY: Check that EFI BIOS ISO boots in qemu
+# LABELS:
+
+set -e
+
+# Source libraries. Uncomment if needed/defined
+#. "${RT_LIB}"
+. "${RT_PROJECT_ROOT}/_lib/lib.sh"
+
+NAME=qemu-raw-efi
+
+clean_up() {
+	rm -rf ${NAME}-*
+}
+trap clean_up EXIT
+
+# see https://github.com/linuxkit/linuxkit/issues/1872 this is very flaky in qemu
+# disabling for now until we fix a config that works
+exit $RT_CANCEL
+
+if command -v qemu-system-x86_64; then
+	if [ ! -f /usr/share/ovmf/bios.bin ]; then
+		exit $RT_CANCEL
+	fi
+fi
+
+linuxkit build --format raw-efi --name "${NAME}" test.yml
+[ -f "${NAME}-efi.img" ] || exit 1
+linuxkit run qemu --uefi "${NAME}-efi.img" | grep -q "Welcome to LinuxKit"
+
+exit 0

test/cases/010_platforms/000_qemu/060_run_raw_efi/test.yml

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+kernel:
+  image: linuxkit/kernel:6.12.52
+  cmdline: "console=ttyS0"
+init:
+  - linuxkit/init:26e546e4ef2669bd44d8c46e3164eb5ba7ae1a5c
+  - linuxkit/runc:98ddef9b5b87cb44783566ed2a86cea95439a631
+onboot:
+  - name: poweroff
+    image: linuxkit/poweroff:4059858a555bed90c2280fa9b060b7a8f8de6d45
+    command: ["/bin/sh", "/poweroff.sh", "10"]

tools/mkimage-raw-efi/README.md

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+# ESP Structure Overview
+
+The script initializes an EFI System Partition using `mkfs.vfat` and populates it with directories and files for **systemd-boot** and a Linux Unified Kernel Image (UKI).
+
+## Partition Layout
+
+```bash
+ESP
+├── EFI
+│   ├── BOOT # contains exactly one of the bootloader binaries below for the respective architecture
+│   │   ├── BOOTX64.EFI # amd64
+│   │   ├── BOOTAA64.EFI # arm64
+│   │   └── BOOTRISCV64.EFI # riscv64
+│   └── Linux
+│       └── linuxkit.efi # LinuxKit Unified Kernel Image (UKI)
+└── loader
+    └── loader.conf # systemd-boot configuration file
+```
+
+UKIs in `EFI/Linux` do not need an explicit entry in `loader/entries` but are automatically picked up by `systemd-boot`.