tools: Add explicit target for NVStore example

[cmonr]

Description

Realized with this PR (#6879) that nvstore was being compiled and tested against all targets. Limited example building for K64F, since example only calls out K64F.

Pull request type

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

[cmonr]

@davidsaada If there are other targets that are explicitly supported, please let us know (similar to this: b61912c).

@ARMmbed/mbed-os-maintainers This could also be why export tests have taken a bit longer, since this example is currently being built against all exported targets instead of a subset.

[davidsaada]

Looks good to me.

[geky]

Looks good to me if David's happy

[cmonr]

/morph build

[cmonr]

Build was stuck trying to start on a node that was never brought online...

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2041
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6919/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1857
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6919/1857

[mbed-ci]

Exporter Build : FAILURE

Build number : 1690
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6919/

[cmonr]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1693
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6919/