U-boot expects uImage format for the kernel image.
In order to LTIB generate a uImage file:
$ export SYSCFG_KTARG=uImage $ ./ltib -p kernel
Setup in U-Boot the kernel bootargs:
u-boot> setenv bootargs noinitrd console=ttymxc0,115200 init=/linuxrc root=/dev/nfs nfsroot=10.29.244.27:/tftpboot/rootfs ip=dhcp
Change 10.29.244.27 to your host IP.
The procedure above is needed when default bootloader used by ltib was redboot.
In some ltib releases (before 2010) default bootloader is u-boot. In this case, ltib will create uImage by default