Fix for serverip issue

U-boot dhcp/bootp client changes "serverip" variable value in case if it
was specified in a dhcp/bootp server configuration.
Renamed serverip var, which now is used for nfsboot target to avoid
updating of this value by DHCP/BOOTP clients and just to use our
pre-defined static value.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3)
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>