esp8266: Enable frozen bytecode, with scripts in modules/ subdir.

To start with, the critical scripts _boot.py and flashbdev.py are frozen
to improve performance and reduce RAM consumption.

Saves about 1000 bytes of heap RAM for a bare boot with filesystem.

esp8266/modules/_boot.py

@@ -0,0 +1,9 @@
+import uos
+from flashbdev import bdev
+
+try:
+    if bdev:
+        vfs = uos.VfsFat(bdev, "")
+except OSError:
+    import inisetup
+    vfs = inisetup.setup()