NRF51 Post-build: Allow Hex BL, SD, APP

[theotherjimmy]

Description

See ARMmbed/mbed-cli#589 for full details.

This PR fixes an issue with the NRF51 post-build script that would fail
when an application was compiled into an Intex Hex format. This prevents
"Address overlap" errors (which are really initial program counter conflicts)
from breaking the build.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2388
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7263/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 2023
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7263/

[cmonr]

Another exporter leaving too soon.
/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2025
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7263/

[mbed-ci]

Test : SUCCESS

Build number : 2176
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7263/2176