NSAPI: Use IPv6 conversion routines from frameworks

[kjbracey]

Description

Share IPv6 conversion code with mbed_trace, Nanostack and
mbed client.

Output formatting is better, being conformant to RFC 5952.

Pull request type

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

[kjbracey]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2314
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7119/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1937
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7119/

[mbed-ci]

Test : SUCCESS

Build number : 2093
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7119/2093