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>

package/fwup/fwup.mk

@@ -10,6 +10,7 @@ FWUP_LICENSE = Apache-2.0
 FWUP_LICENSE_FILES = LICENSE
 FWUP_DEPENDENCIES = libconfuse libarchive libsodium
 FWUP_AUTORECONF = YES
+FWUP_CONF_ENV = ac_cv_path_HELP2MAN=""
 
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))