ports/unix: Make MICROPY_PY_SYS_EXECUTABLE overridable.

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

ports/unix/mpconfigport.h

@@ -155,7 +155,9 @@ typedef long mp_off_t;
 #define MICROPY_PY_SYS_PATH_ARGV_DEFAULTS (0)
 
 // Enable sys.executable.
+#ifndef MICROPY_PY_SYS_EXECUTABLE
 #define MICROPY_PY_SYS_EXECUTABLE (1)
+#endif
 
 #define MICROPY_PY_SOCKET_LISTEN_BACKLOG_DEFAULT (SOMAXCONN < 128 ? SOMAXCONN : 128)