tools, NRF51-post-build: Use paths to hex files
tools, NRF51-post-build: Use paths to hex files

### Description

The NRF51 post build hook finds it's hex files using the Resources
object. This PR fixes the corner case where the hex files within a
project have a name!=path (This really only happens online).

### Pull request type

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

kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,mbedmicro/mbed