azure-netxduo-demo: Add fixes for CS310-FPGA

This commit does the following:

- Increases the dns server address buffer size
as in case of CS310-FPGA a buffer too small
error was encountered when executing
nx_dhcp_interface_user_option_retrieve() function.

- Adds busy wait code for a ceratin period of time so
the ethernet link comes up as in case of CS310-FPGA
it may takes some time for the link to come up unlike
the FVP which is way faster.

Signed-off-by: Ahmed.Ismail <Ahmed.Ismail@arm.com>