dnsmasq: Make the --dhcp-host logic easier to understand

Use an if/else statement to cover the two different syntaxes.  Add
comments explaining what the end results should look like.

This patch should not change the script's output.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>

Backport of r42320

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42390 3c298f89-4303-0410-b956-a3cf2f4a3e73