rp2/rp2_flash: Workaround multicore lockout not being reset.

With regression test.

See upstream bug https://github.com/raspberrypi/pico-sdk/issues/2201

Tested-by: Angus Gratton <angus@redyak.com.au>
Signed-off-by: Mike Bell <mdb036@gmail.com>

tests/ports/rp2/rp2_thread_reset_part1.py

@@ -0,0 +1,18 @@
+# This is a regression test for https://github.com/micropython/micropython/issues/16619
+# it runs in two parts by necessity:
+#
+# - This "part1" creates a non-terminating thread.
+# - The test runner issues a soft reset, which will terminate that thread.
+# - "part2" is the actual test, which is whether flash access works correctly
+#   after the thread was terminated by soft reset.
+
+import _thread
+
+
+def infinite():
+    while True:
+        pass
+
+
+_thread.start_new_thread(infinite, ())
+print("Part 1 complete...")

tests/ports/rp2/rp2_thread_reset_part1.py.exp

@@ -0,0 +1 @@
+Part 1 complete...

tests/ports/rp2/rp2_thread_reset_part2.py

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+# This is part2 of a two-part regression test, see part1
+# for details of what's expected.
+import os
+
+FILENAME = "/rp2_thread_reset_test.txt"
+
+print("Starting")
+with open(FILENAME, "w") as f:
+    f.write("test")
+print("Written")
+os.unlink(FILENAME)
+print("Removed")

tests/ports/rp2/rp2_thread_reset_part2.py.exp

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+Starting
+Written
+Removed