nss: fix parallel build

There is a hack for building subdirectories before the parent
directory. There is a bug in this which causes the EXPORTS and
PRIVATE_EXPORTS targets to be built twice leading to a race condition.

Change-Id: I94230722671aa6c0c10afdfe376bbb3fa86d109f

libs/nss/patches/004-fix_parallel_build.patch

+diff --git a/coreconf/rules.mk b/coreconf/rules.mk
+index dc72e9cef..fe7acc45d 100644
+--- a/nss/coreconf/rules.mk
++++ b/nss/coreconf/rules.mk
+@@ -38,6 +38,8 @@ ifndef IGNORE_DIRS
+ ifneq (,$(filter .,$(DIRS)))
+ TARGETS = $(NULL)
+ ALL_TRASH = $(NULL)
++EXPORTS = $(NULL)
++PRIVATE_EXPORTS = $(NULL)
+ endif
+ 
+ $(DIRS):
+@@ -52,6 +54,8 @@ endif
+ 
+ export: $(DIRS) private_export
+ 
++private_export: $(DIRS)
++
+ release_export: $(DIRS)
+ 
+ libs program install: $(DIRS) $(TARGETS)