With upstream commit
https://git.kernel.org/cgit/network/ethtool/ethtool.git/commit/?id=875616dfcbe57ea0f639a20d85fcbad2172ad744

there is now an option to produce a smaller ethtool build which will
disable pretty printing (Ethernet drivers, SFP diagnostics...) for
platforms that do not need it.

Hook a menu configuration option to control that option. Build size
differences on ar71xx:

With:
-rw-r--r-- 1 florian florian 79K mai   23 10:43
bin/ar71xx/packages/packages/ethtool_3.18-1_ar71xx.ipk

Without:
-rw-r--r-- 1 florian florian 23K mai   23 10:43
bin/ar71xx/packages/packages/ethtool_3.18-1_ar71xx.ipk

Signed-off-by: Florian Fainelli <florian@openwrt.org>