fwup: disable documentation build

fwup: disable documentation build

Documentation build with help2man relies on the output of running 'fwup
--help'. That obviously can't work with cross compilation.

Fixes:
http://autobuild.buildroot.net/results/bdd/bdd8137973c37824f6aa9261ee307f6ed21b9166/
http://autobuild.buildroot.net/results/96a/96a53b3ec666bd09f09c8a3e5b31eff73e3dd0fa/
http://autobuild.buildroot.net/results/e0b/e0bcfaf9dd36d4992e507d18afeb1cec56048987/

Cc: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>