kernel: select work queue implementation

Attempts to reimplement the existing work API using a new work
implementation failed, primarily due to heavy use of whitebox testing
in validating the original API.  Add a temporary Kconfig that will
select between the two implementations so we can use the same
identifiers but select which implementation they reference.

This commit just adds the selection infrastructure and uses it to
conditionalize the existing implementation in anticipation of the new
one in the next commit.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>