Fix Makefile export with Python 3

[lassebm]

Description

In Python 3, the map() function returns a map object, not a list object as in
Python 2. Ensure a list object is returned from format_flags() by wrapping
map() in list(). This is compatible with both Python 2 and 3.

Regression in c777bd6.

Pull request type

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

[0xc0170]

Thank you @lassebm for fix , we already assigned reviewers and will review it soon !

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3523
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8577/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3140
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8577/

[cmonr]

/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 3307
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8577/3307

[NirSonnenschein]

@theotherjimmy , care to take a look? this really should only take a moment (tiny change)

[theotherjimmy]

@NirSonnenschein Thanks for the ping. Feel free to merge this.