build: Always run checkapi when building system image

This should significantly reduce the number of quail stars given out.

This only applies when building the system image.  Packages may still
be built and tested individually without any impact.

If you like to live dangerously, set I_WANT_A_QUAIL_STAR in your
environment.

TICKET: CYNGNOS-1950
Change-Id: I3da6646f022f2e967ebe2c3dd9ed616fab7991fe
(cherry picked from commit 470b95c0)