NetsecDxe: SnpInitialize() waits for media linking up

The latest NetsecDxe requires issueing phy reset at the
last stage of initialization to safely exit loopback mode.
However, as a result, it takes a couple of seconds for link state
to get stable, which could cause auto-chosen pxeboot to fail
due to MediaPresent check error.

This patch adds link state check with 5s timeout in NetsecDxe
initialization. The timeout value can be adjustable via
configuration file.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>