Take OUTPUT_EXT into account in managed bl

[theotherjimmy]

Resolves #6005

@marcuschangarm

[theotherjimmy]

Managed bl on K64F

mbed_app.json addition:

    "target_overrides": {                                                                                
        "*": {                                                                                           
            "target.OUTPUT_EXT": "hex"                                                                   
        }     

before

Merging Regions:
  Filling region bootloader with /home/jimmy/src/armmbed/mbed-os-example-bootloader-blinky/bootloader/K64F.bin
  Filling region application with ./BUILD/k64f/gcc_arm/mbed-os-example-bootloader-blinky_application.hex
Space used after regions merged: 0x2bf90
Text 44.4KB Data 2.61KB BSS 8.83KB ROM 47KB RAM 11.4KB
Image: ./BUILD/k64f/gcc_arm/mbed-os-example-bootloader-blinky.bin

after

Merging Regions:
  Filling region bootloader with /home/jimmy/src/armmbed/mbed-os-example-bootloader-blinky/bootloader/K64F.bin
  Filling region application with ./BUILD/k64f/gcc_arm/mbed-os-example-bootloader-blinky_application.hex
Space used after regions merged: 0x2bf90
Text 44.4KB Data 2.61KB BSS 8.83KB ROM 47KB RAM 11.4KB
Image: ./BUILD/k64f/gcc_arm/mbed-os-example-bootloader-blinky.hex

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1057
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6007/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 736
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6007/

[mbed-ci]

Test : SUCCESS

Build number : 863
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6007/863