after a lot of trial and error i am able to install my final boot image to SSD (mSATA). I tried a lot of combinations with different linux boot images from SDCARD or USB stick, nothing works. The final solution with a systemrescuecd, dhcp and network:
- Install the current systemrescuecd on USB stick as described in http://www.system-rescue-cd.org/Sysresccd-manual-en_How_to_install_SystemRescueCd_on_an_USB-stick
- Boot to bootloader, select default entry and press tab
- remove all stuff after the "--" and add rootpass=123456 (you need a password to login via ssh)
- wait 40-60 seconds, get the current ip address from your dhcp server logs
- ssh login as root
You can now install all your stuff without serial console trouble.
I'm using grub2 booting an custom ISO image.