Fix mkimage script to support make and idf.py builds

[fadushin]

This change set allows the mkimage script to work with both make and idf.py builds.

Signed-off-by: Fred Dushin [email protected]

These changes are made under both the "Apache 2.0" and the "GNU Lesser General Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later