okay, I think I found a workaround
partition the disk manually (I used a single partition with the default linux type 0x83)
sudo su
copy the (wrongly named) dCore<jessie,etc...>.gz and vmlinuz<jessie,etc...> files to root. I found mine in /mnt/sdb1/boot
sce-import dCore-install
sce-load dCore-install
dCore-install
select frugal install to the partition file system type ext2 and add your boot flags
reboot and install your stuff