kernel: Add more threads to kernel build process

Change-Id: I9c90de7820308542c7bb15605df519976dd04e0b
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index 74f3413..e16c47e 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -95,6 +95,9 @@
 # Clear this first to prevent accidental poisoning from env
 KERNEL_MAKE_FLAGS :=
 
+# Add back threads, ninja cuts this to $(nproc)/2
+KERNEL_MAKE_FLAGS += -j$$(nproc)
+
 ifeq ($(KERNEL_ARCH),arm)
   # Avoid "Unknown symbol _GLOBAL_OFFSET_TABLE_" errors
   KERNEL_MAKE_FLAGS += CFLAGS_MODULE="-fno-pic"