net/dhcpv4: Refactor send_discover
net/dhcpv4: Refactor send_discover

Refactor send_discover() to provide a clean separation between state
machine and action.

Change-Id: I96510cc6a7ab16216821fe65ea4289a78f6443aa
Signed-off-by: Marcus Shawcroft <cf6354583ee83038f2010cbcbe1b1f2adb85107d@arm.com>

fbsder/zephyr,fbsder/zephyr,fbsder/zephyr,fbsder/zephyr,fbsder/zephyr