rockboxdev: Make it work on macOS.

macOS doesn't know about nproc; sysctl is the tool to be used on macOS.

Change-Id: Ic7dcf9be51d7ef7b52394ebb2a4df1dc3e31097b

tools/rockboxdev.sh

@@ -32,7 +32,12 @@ fi
 # This is the absolute path to where the script resides.
 rockboxdevdir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
 
-parallel=`nproc`
+if [ `uname -s` = "Darwin" ]; then
+    parallel=`sysctl -n hw.physicalcpu`
+else
+    parallel=`nproc`
+fi
+
 if [ $parallel -gt 1 ] ; then
   make_parallel=-j$parallel
 fi