modsys: Add optional support for sys.platform.
Ports which wants to have it, should define MICROPY_PY_SYS_PLATFORM to a
string value they need.
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index ace7a41..bf2cf73 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -43,6 +43,7 @@
#define MICROPY_OPT_COMPUTED_GOTO (1)
#define MICROPY_PY_BUILTINS_FROZENSET (1)
#define MICROPY_PY_SYS_EXIT (1)
+#define MICROPY_PY_SYS_PLATFORM "linux"
#define MICROPY_PY_SYS_STDFILES (1)
#define MICROPY_PY_CMATH (1)
#define MICROPY_PY_IO_FILEIO (1)