RTEMS: use DHCP network configuartion for shell example.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
RTEMS: alternative example network configuration for ne2k card under QEMU.

This allows to run example build for pc686 board in QEMU
with networking

qemu-system-i386 -kernel _compiled/pc686/bin/orte_rtems_shell_example \
  -net nic,vlan=0,model=ne2k_isa \
  -net tap,script=/etc/qemu-ifup

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
RTEMS: QEMU provides ETHERNET HW address so there is no need to setup it in application.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>