Use `nproc` processing units instead of 64 as default

The variable J is used as number of jobs to parallelize the kernel
build.  In uninitialized, this patch sets J as the number of processing
units available in the current system, instead of a fixed number of 64.

Change-Id: I825f50f0b904474b699f883066249537162a15d0
Signed-off-by: Alessio Balsini <balsini@google.com>
diff --git a/net/test/run_net_test.sh b/net/test/run_net_test.sh
index 01130bb..58a0e15 100755
--- a/net/test/run_net_test.sh
+++ b/net/test/run_net_test.sh
@@ -89,7 +89,7 @@
 
 # Parse arguments and figure out which test to run.
 ARCH=${ARCH:-um}
-J=${J:-64}
+J=${J:-$(nproc)}
 MAKE="make"
 OUT_DIR=$(readlink -f ${OUT_DIR:-.})
 KERNEL_DIR=$(readlink -f ${KERNEL_DIR:-.})