ARM: bootscript: Remove hard-coded rootpath from K+P's tpcboot.cmd

The 'rootpath' can be provided by DHCP server. Removing this line prevents
from overwriting it.

To do that on the DHCP server side (isc-dhcp-server) modify dhcpd.conf
and add 'option root-path "192.168.X.Y:/srv/tftp/rootfs";'

Signed-off-by: Lukasz Majewski <lukma@denx.de>