dropbear: Store the hostkey persistently

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-ohos-core/recipes-core/dropbear/dropbear_%.bbappend

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+inherit writables
+
+WRITABLES = "hostkeys"
+WRITABLE_PATH[hostkeys] = "/etc/dropbear"