bind: fix intermittent build issues with high BR2_JLEVEL

Build sometimes breaks with:

libtool: link: `unix/os.lo' is not a valid libtool object
make[3]: *** [rndc-confgen] Error 1
make[3]: *** Waiting for unfinished jobs....
make[4]: Leaving directory `/scratch/peko/build/bind-9.6-ESV-R4/bin/rndc/unix'

So disable parallel builds.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/bind/bind.mk

@@ -6,6 +6,7 @@
 
 BIND_VERSION = 9.6-ESV-R4
 BIND_SITE = ftp://ftp.isc.org/isc/bind9/$(BIND_VERSION)
+BIND_MAKE = $(MAKE1)
 BIND_TARGET_SBINS = lwresd named named-checkconf named-checkzone
 BIND_TARGET_SBINS += named-compilezone rndc rndc-confgen dnssec-dsfromkey
 BIND_TARGET_SBINS += dnssec-keyfromlabel dnssec-keygen dnssec-signzone