nut: disable parallel builds

The build system has a race condition causing build failure,
so ensure make -j1 is used.

Fixes:
http://autobuild.buildroot.net/results/0dd/0ddd856bcbec2db0500791fd428ba053d6e4fa1b/build-end.log

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/nut/nut.mk

@@ -14,6 +14,9 @@ NUT_DEPENDENCIES = host-pkgconf
 # Our patch changes m4 macros, so we need to autoreconf
 NUT_AUTORECONF = YES
 
+# Disable parallel builds
+NUT_MAKE = $(MAKE1)
+
 # Put the PID files in a read-write place (/var/run is a tmpfs)
 # since the default location (/var/state/ups) maybe readonly.
 NUT_CONF_OPT = \