ethtool: Allow enabling/disabling pretty printing

Ethtool has a configure script option allowing the enabling or disabling of
pretty printing for drivers, SFP modules etc which can save a significant
amount of bytes on the root filesystem. Allow selecting that, and turn it on by
default (like ethtool's configure script).

[Peter: fix Config.in indentation/style]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>