tools/pyboard.py: Support executing .mpy files directly.

This patch allows executing .mpy files (including native ones) directly on
a target, eg a board over a serial connection.  So there's no need to copy
the file to its filesystem to test it.

For example:

    $ mpy-cross foo.py
    $ pyboard.py foo.mpy

tools/pyboard.py

@@ -484,6 +484,33 @@ def filesystem_command(pyb, args):
         pyb.close()
         sys.exit(1)
 
+_injected_import_hook_code = """\
+import uos, uio
+class _FS:
+  class File(uio.IOBase):
+    def __init__(self):
+      self.off = 0
+    def ioctl(self, request, arg):
+      return 0
+    def readinto(self, buf):
+      buf[:] = memoryview(_injected_buf)[self.off:self.off + len(buf)]
+      self.off += len(buf)
+      return len(buf)
+  mount = umount = chdir = lambda *args: None
+  def stat(self, path):
+    if path == '_injected.mpy':
+      return tuple(0 for _ in range(10))
+    else:
+      raise OSError(-2) # ENOENT
+  def open(self, path, mode):
+    return self.File()
+uos.mount(_FS(), '/_')
+uos.chdir('/_')
+from _injected import *
+uos.umount('/_')
+del _injected_buf, _FS
+"""
+
 def main():
     import argparse
     cmd_parser = argparse.ArgumentParser(description='Run scripts on the pyboard.')
@@ -544,6 +571,9 @@ def main():
         for filename in args.files:
             with open(filename, 'rb') as f:
                 pyfile = f.read()
+                if filename.endswith('.mpy') and pyfile[0] == ord('M'):
+                    pyb.exec_('_injected_buf=' + repr(pyfile))
+                    pyfile = _injected_import_hook_code
                 execbuffer(pyfile)
 
         # exiting raw-REPL just drops to friendly-REPL mode