Consistently use nproc for counting the CPUs

Coreutils command nproc can be used on Linux and BSD to count the number of
available CPU cores. Use this instead of relying on the parsing of the
Linux specific proc file system.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/test/fs/fs-test.sh b/test/fs/fs-test.sh
index 721af71..b877481 100755
--- a/test/fs/fs-test.sh
+++ b/test/fs/fs-test.sh
@@ -87,7 +87,7 @@
 # Generate sandbox U-Boot - gleaned from /test/dm/test-dm.sh
 function compile_sandbox() {
 	unset CROSS_COMPILE
-	NUM_CPUS=$(cat /proc/cpuinfo |grep -c processor)
+	NUM_CPUS=$(nproc)
 	make O=sandbox sandbox_config
 	make O=sandbox -s -j${NUM_CPUS}