Add combined hex target to CMake

[kaidokert]

Description

Adding a combined-hex build target to CMake exporter, copied from Make exporter. Makes working with Nordic softdevices easier

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[x] Feature
[ ] Breaking change

[cmonr]

Neat!

[cmonr]

@thinkberg Would you mind taking a look, since you were the last to update the CMake exporter?

[cmonr]

/morph build

[thinkberg]

very nice. looks good.

[mbed-ci]

Build : SUCCESS

Build number : 2852
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7762/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2483
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7762/

[cmonr]

/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 2612
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7762/2612

[DBS06]

Iis it possible to write with this change a custom-mbed bootloader for Nordic Targets with SoftDevice? Because, until now the SoftDevice is added as target.bootloader_img:.