[scripts] update do-armemu-test to create a default block device file if it doesn't already exist

scripts/do-armemu-test

@@ -2,6 +2,10 @@
 
 export PROJECT=armemu-test
 
-make -C ../armemu && 
-make && 
+if [ ! -f blk.bin ]; then
+	dd if=/dev/zero of=blk.bin bs=1024k count=16
+fi
+
+make -j8 -C ../armemu &&
+make -j8 &&
 (cd build-$PROJECT; ../../armemu/build-generic/armemu)