Disabled by default; enable with `export SECRETKEEPER_ENABLED=y` before
building.
Also needs the Secretkeeper TA to be present in Trusty; if the TA is
absent, the HAL service will (repeatedly) fail to connect.
Test: build, VtsSecretkeeperTargetTest
Bug: 306364873
Change-Id: I529013395d0e3afbff4a24b663088adce2a23805