Add support for Mbed-OS 6.15.0 (#4909)

The default Mbed OS device (STM32F4) had been changed to FRDM-K64F that is
supported by both Mbed OS 5 and Mbed OS 6 versions. Makefile had also been
modified to have a flash recipe.

JerryScript-DCO-1.0-Signed-off-by: Roland Takacs roland.takacs@h-lab.eu

targets/os/mbedos/Makefile.travis

@@ -0,0 +1,38 @@
+# Copyright JS Foundation and other contributors, http://js.foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+# Default target for running the build test outside the Travis CI environment.
+all:
+	$(MAKE) install
+	$(MAKE) script
+
+
+## Targets for installing build dependencies of the Mbed OS JerryScript target.
+
+install-mbedos:
+	git clone https://github.com/ARMmbed/mbed-os.git ../mbed-os -b mbed-os-6.15.0
+
+# Deploy Mbed and install Mbed Python dependencies.
+install-mbedos-deps:
+	pip install mbed-cli
+	pip install -r ../mbed-os/requirements.txt
+
+install: install-mbedos install-mbedos-deps
+
+## Targets for building Mbed OS with JerryScript.
+
+# Build the firmware (Mbed OS with JerryScript).
+script:
+	$(MAKE) -C targets/os/mbedos MBED_OS_DIR=$(realpath ../mbed-os)