tools/run_guest_kernel.sh: Support concatenated initramfs images

This allows booting a kernel with the firmware cpio and a cpio
containing matching modules. The modules initramfs can be prepared from
the kernel source directory with:

`make dir-pkg && usr/gen_initramfs.sh -o initramfs_mod.cpio -u squash -g squash ./tar-install/lib/`

The modules are available in /modules/$(uname -r) and the initramfs can
handle modules in the same way as the vendor firmware to make them
available for itself and the main OS.

Signed-off-by: Janne Grunau <j@jannau.net>

proxyclient/tools/run_guest_kernel.sh

@@ -47,6 +47,14 @@ else
 fi
 
 if [ -n "$initramfs" ]; then
+    initramfs_size=$(stat --printf='%s' "$initramfs")
+    python - << EOF >>"$TMPDIR/m1n1-linux.bin"
+import os, sys
+
+magic = b'm1n1_initramfs'
+size = int(${initramfs_size}).to_bytes(4, byteorder='little')
+os.write(sys.stdout.fileno(), magic + size)
+EOF
     cat "$initramfs" >>"$TMPDIR/m1n1-linux.bin"
 fi