examples: add Linux test image

examples/linux/README.md

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+# Linux artifacts for eVisor's tests
+
+## How to build Linux image
+
+See also https://zenn.dev/hidenori3/articles/5db71181a33d56 (Japanese)
+
+### Sync source tree
+
+```shell
+git clone --depth=1 -b v6.8 https://github.com/torvalds/linux.git
+cd linux
+```
+
+### Build 
+
+Create `arch/arm64/configs/qemu-busybox-min.config`:
+
+```
+CONFIG_SERIAL_AMBA_PL011=y
+CONFIG_SERIAL_AMBA_PL011_CONSOLE=y
+
+CONFIG_GPIOLIB=y
+CONFIG_GPIO_PL061=y
+
+CONFIG_KEYBOARD_GPIO=y
+
+CONFIG_CMDLINE="console=ttyAMA0 nokaslr rdinit=/sbin/init"
+```
+
+```shell
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make allnoconfig
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make qemu-busybox-min.config
+```
+
+```shell
+ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j$(nproc)
+```
+
+### Generate ROM file from the Image
+
+```shell
+xxd -i arch/arm64/boot/Image > qemu_rom.c
+```