lib: updatehub: Kconfig: Add coap max retry option

The current implementation uses a fixed value for max retries. That
value could be good for an wired network like Ethernet. However,
wireless network can suffer with higher packet collision, low reception
signal etc. This refacts the variable to be defined at Kconfig. This
way max retries can be adjust conform the current media.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>