net/dhcpv4: Refactor SELECTING state entry.
net/dhcpv4: Refactor SELECTING state entry.

Factor out common code associated with entering the SELECTING state.

Change-Id: If34929e5f13dff7650388a2b491a01cbbf5dc100
Signed-off-by: Marcus Shawcroft <cf6354583ee83038f2010cbcbe1b1f2adb85107d@arm.com>

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