Adding file type for linker script arg in make script

[bridadan]

Description

When supplying an external linker script, the tools currently traceback:

$ mbed compile --linker targets\TARGET_Freescale\TARGET_MCUXpresso_MCUS\TARGET_MCU_K64F\device\TOOLCHAIN_GCC_ARM\MK64FN1M0xxx12.ld
[mbed] Auto-installing missing Python modules...
c:\python27\lib\site-packages\requests\__init__.py:80: RequestsDependencyWarning: urllib3 (1.23) or chardet (3.0.4) doesn't match a supported version!
  RequestsDependencyWarning)
Building project mbed (NUCLEO_F411RE, GCC_ARM)
Scan: mbed
Scan: env
[ERROR] add_file_ref() takes exactly 4 arguments (3 given)
[mbed] ERROR: "c:\python27\python.exe" returned error.
       Code: 1
       Path: "C:\Users\bridan01\onedrive_arm\Documents\dev\m_mbed\mbed"
       Command: "c:\python27\python.exe -u C:\Users\bridan01\onedrive_arm\Documents\dev\m_mbed\mbed\tools\make.py -t GCC_ARM -m NUCLEO_F411RE --source . --build .\BUILD\NUCLEO_F411RE\GCC_ARM --linker targets\TARGET_Freescale\TARGET_MCUXpresso_MCUS\TARGET_MCU_K64F\device\TOOLCHAIN_GCC_ARM\MK64FN1M0xxx12.ld"
       Tip: You could retry the last command with "-v" flag for verbose output

This patch fixes the traceback.

Pull request type

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

[cmonr]

So, funny thing.
This needs a rebase :/

[bridadan]

Rebased!

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3434
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8495/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3054
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8495/

[mbed-ci]

Test : SUCCESS

Build number : 3223
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8495/3223