nettle: enable fat build

This allows to include optimizations such as ARM neon which
are detected on run-time.

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>
[Jo-Philipp Wich: picked from openwrt#191 and rebased onto LEDE master]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>