unix/mpconfigport.mk: Document MICROPY_STANDALONE make-level option.

Avoid using system libraries, use copies bundled with MicroPython as
submodules (currently affects only libffi, other dependencies either
already used as bundled-only (axtls), or can't be bundled (so far),
like libjni).

unix/mpconfigport.mk

@@ -26,3 +26,7 @@ MICROPY_PY_USSL = 1
 
 # jni module requires JVM/JNI
 MICROPY_PY_JNI = 0
+
+# Avoid using system libraries, use copies bundled with MicroPython
+# as submodules (currently affects only libffi).
+MICROPY_STANDALONE = 0