Thank you for your fast answer!
I tried it, but it doesn't work for me.
The good news is, that I fixed the problem. The u-boot parameters for Linux were wrong.
To change them, connect the Marsboard over the debug interface with your PC (Linux: minicom, Windows: Hyperterminal).
The following parameters worked for me:
bootargs_nfs=setenv bootargs ${bootargs} root=/dev/nfs ip=dhcp nfsroot=${serverip}:${nfsroot},v3,tcp enable_wait_mode=off
bootcmd_net=dhcp; run bootargs_nfs;bootm
bootargs_mmc0=setenv bootargs ${bootargs} root=/dev/mmcblk0p1 rootwait rw
bootargs_mmc1=setenv bootargs ${bootargs} root=/dev/mmcblk1p1 rootwait rw
bootcmd_mmc0=run bootargs_mmc0;mmc dev 1;mmc read ${loadaddr} 0x800 0x2000;bootm
bootcmd_mmc1=run bootargs_mmc1;mmc dev 1;mmc read ${loadaddr} 0x800 0x2000;bootm
bootcmd=if mmc dev 0; then run bootcmd_mmc1; else run bootcmd_mmc0; fi
clearenv=sf probe 0 && sf erase 0xc0000 0x2000 && echo restored environment to factory default
upgradeu=for disk in 0 1 ; do mmc dev ${disk} ;for fs in fat ext2 ; do ${fs}load mmc ${disk}:1 10008000 /6q_upgrade && source 10008000 ; done ; done
bootfile=_BOOT_FILE_PATH_IN_TFTP_
nfsroot=_ROOTFS_PATH_IN_NFS_
ethact=FEC0
bootargs=console=ttymxc1,115200 video=mxcfb0:dev=hdmi,1280x960M@60,if=RGB24
Remark: Depending on your display, you have to change the resolution under 'bootargs' (1280x960M@60).