The piCore default is: $ cat /mnt/mmcblk0p1/cmdline.txt
zswap.compressor=lz4 zswap.zpool=z3fold console=tty1 root=/dev/ram0 elevator=deadline rootwait quiet nortc loglevel=3 noembed
..you appear to be using: coherent_pool=1M 8250.nr_uarts=0 snd_bcm2835.enable_compat_alsa=0 snd_bcm2835.enable_hdmi=1 bcm2708_fb.fbwidth=720 bcm2708_fb.fbheight=480 bcm2708_fb.fbswap=1 vc_mem.mem_base=0x3ec00000 vc_mem.mem_size=0x40000000 zswap.compressor=lz4 zswap.zpool=z3fold console=/dev/null root=/dev/ram0 elevator=deadline rootwait quiet nortc loglevel=3 noembed lang=de_DE.UTF-8 kmap=de waitusb=5 logo.nologo=1 vga=795
I would suggest using xf86-video-fbdev instead of xf86-video-fbturbo, remove bcm2708_fb.fbwidth=720, bcm2708_fb.fbheight=480, bcm2708_fb.fbswap=1, vc_mem.mem_base=0x3ec00000 vc_mem.mem_size=0x40000000 and vga=795 and try again.