EPG5000 uses ath79 nvram functions, but does not select it. This breaks
build if no other board support enabling nvram support is selected.

Closes #19952.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46190 3c298f89-4303-0410-b956-a3cf2f4a3e73