tools: do not build bc documentation (#13841)

Building the "bc" documentation requires the "makeinfo" program to be
installed on the host.

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

git-svn-id: svn://svn.openwrt.org/openwrt@37209 3c298f89-4303-0410-b956-a3cf2f4a3e73