add a script to build examples in background

dev_env/1_pico_sdk/3_background_build_uf2s.sh

@@ -0,0 +1,22 @@
+#! /usr/bin/env bash
+
+set -e
+
+echo ""
+echo "Setting environment variables"
+source ../set_pico_envars
+
+echo "This starts up background jobs to rebuild all the example uf2s."
+echo "It starts the jobs and then exits, leaving them running."
+echo "You can do other stuff or just monitor their progress by doing"
+echo ""
+echo "    tail -f"
+echo ""
+echo "on the logfiles. This takes about half an hour on my 32 GB"
+echo "Acer Nitro 5 laptop. It probably requires at least 8 GB of"
+echo "RAM."
+
+/usr/bin/time ./build_rp2040_examples.sh > build_rp2040_examples.log 2>&1 &
+/usr/bin/time ./build_rp2350_examples.sh > build_rp2350_examples.log 2>&1 &
+
+echo "Finished"