Add initial checkapi / API .txt generation

Wire up .txt signature generation for the core platform
API.

Testing:
make core-platform-api-stubs-check-last-released-api
make core-platform-api-stubs-update-current-api

Test: See above
Change-Id: Icfe08a8416deaab5b3466b78d7e2536622a2c465
5 files changed