Hi,
i finally figured out, how to boot a legacy linux:
I enabled the "bootable"-flag of the boot partition (although grub was installed on MBR). Now the "Legacy"-option in the F9-menu appears for my usbstick and booting is possible.
After removing the "bootable"-flag, the stick won't be recognized again.
My previously owned laptops - HP 1020 G1, Thinkpad X220 - would boot even without the bootable-flag - that's why it took me some testing to solve the mystery.
HPUser2k15