PSOC6: enable export to CMake
PSOC6: enable export to CMake

The approach for the hex_files subset selection is identical
to makefile exporter: https://github.com/ARMmbed/mbed-os/pull/9466

Single hex file should be passed to srec_cat when hex_filename
is set in targets.json or mbed_app.json.

mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os