If we build everything correctly with multiple builds, and an
ERR directory had been previously created, we failed to report
that everything was fine because grep failed to find anything
in the ERR directory. Use grep -r, which doesn't complain if
there are no input files.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Tested-by: Tom Rini <trini@ti.com>