ports/unix: Make MICROPY_PY_OS_SYSTEM overridable.

Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>

ports/unix/variants/mpconfigvariant_common.h

@@ -91,7 +91,9 @@
 #define MICROPY_PY_OS_ERRNO            (1)
 #define MICROPY_PY_OS_GETENV_PUTENV_UNSETENV (1)
 #define MICROPY_PY_OS_SEP              (1)
+#ifndef MICROPY_PY_OS_SYSTEM
 #define MICROPY_PY_OS_SYSTEM           (1)
+#endif
 #define MICROPY_PY_OS_URANDOM          (1)
 
 // Enable the unix-specific "time" module.