board/BuR/common: increase NET_RETRY_COUNT to 10

Sometimes boards may need more time to become stable network connection
due to several reasons:

- phy speed
- link-partner (switch)

Therefore we increase the retry-count to 10 for making sure that network
connection works always.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Tom Rini <trini@konsulko.com>