In order to increase the speed of bitbake compiling (use two cpu in the same time), go to the 'overo-oe/build/conf' folder. Open file 'site.conf'.
un-comment following statement:
# Uncomment these lines to enable parallel make.
# This allows make to spawn mutliple processes to take advantage of multiple
# processors. Useful on SMP machines
PARALLEL_MAKE = "-j 2"
#BB_NUMBER_THREADS = "4"
done! both cpus are running at over 90% speed.