unix: minimal: Don't use readline support.

After switching to builtin readline support, "minimal" no longer builds, and
minimal doesn't really need readline support.

unix/Makefile

@@ -141,7 +141,7 @@ fast:
 
 # build a minimal interpreter
 minimal:
-	$(MAKE) COPT="-Os -DNDEBUG" CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_minimal.h>"' BUILD=build-minimal PROG=micropython_minimal MICROPY_PY_TIME=0 MICROPY_PY_TERMIOS=0 MICROPY_PY_SOCKET=0 MICROPY_PY_FFI=0
+	$(MAKE) COPT="-Os -DNDEBUG" CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_minimal.h>"' BUILD=build-minimal PROG=micropython_minimal MICROPY_PY_TIME=0 MICROPY_PY_TERMIOS=0 MICROPY_PY_SOCKET=0 MICROPY_PY_FFI=0 MICROPY_USE_READLINE=0
 
 # build an interpreter for coverage testing and do the testing
 coverage: