I had done by editing /syslinux.cfg file inside bootable USB flash drive.
My device is PC Engines APU2, it doesn't use EFI boot.
- Write "install-image-7.x.img" to USB Flash Drive. (don't write ".iso" file)
- Mount USB-FD on Linux. (Windows can't mount this USB-FD.)
- Edit "syslinux.cfg" file.
After editing,
$ cat /mnt/syslinux.cfg
SERIAL 0 115200
CONSOLE 0
default system
LABEL system
KERNEL linux
APPEND console=ttyS0,115200n8 load_ramdisk=1 -install -hdd
This works for me.