nano: avoid creating system.img in build

flash.sh was creating a system.img when creating the SD card image,
avoid that. This also prevents a bug in flash.sh which creates a loop
device and does not remove it after finishing.

build-core.sh

@@ -82,6 +82,7 @@ if [ "$board" = nano ]; then
     rm jetson.img
     cd Linux_for_Tegra
     patch -p1 < ../../tarball-parts/nano/create-jetson-nano-sd-card-image.sh.patch
+    patch -p1 < ../../tarball-parts/nano/flash.sh.patch
     sudo ./create-jetson-nano-sd-card-image.sh -o jetson.img -s 600M -r 200
     popd
     mv "$outdir"/Linux_for_Tegra/jetson.img "$final_tree"

tarball-parts/nano/create-jetson-nano-sd-card-image.sh.patch

@@ -10,6 +10,15 @@
  	fi
  }
  trap cleanup EXIT
+@@ -148,7 +148,7 @@
+ 	echo "${script_name} - creating signed images"
+
+ 	# Generate flashcmd.txt for signing images
+-	BOARDID="3448" FAB="${rev}" "${l4t_dir}/flash.sh" "--no-flash" "p3448-0000-sd" "mmcblk0p1"
++	BOARDID="3448" FAB="${rev}" "${l4t_dir}/flash.sh" "-r" "--no-flash" "p3448-0000-sd" "mmcblk0p1"
+
+ 	if [ ! -f "${bootloader_dir}/flashcmd.txt" ]; then
+ 		echo "ERROR: ${bootloader_dir}/flashcmd.txt not found" > /dev/stderr
 @@ -169,7 +171,12 @@
  {
  	echo "${script_name} - create partitions"

tarball-parts/nano/flash.sh.patch

@@ -0,0 +1,13 @@
+--- a/flash.sh	2019-03-13 08:45:13.000000000 +0100
++++ b/flash.sh	2019-04-15 14:46:38.445259529 +0200
+@@ -1873,10 +1873,6 @@
+ 	APP_TAG+="-e s/APPFILE/${localsysfile}/ ";
+ 	if [ "${skip_systemimg}" != "true" ]; then
+ 		echo "Reusing existing ${localsysfile}... ";
+-		if [ ! -e "${localsysfile}" ]; then
+-			echo "file does not exist.";
+-			exit 1;
+-		fi;
+ 		echo "done.";
+ 	else
+ 		echo "Skip generating ${localsysfile}";