Trying to decrease unit test execution time on embedded systems

@mortenvp @jpihl I managed to significantly decrease the unit test execution times on our slow embedded builders with this little change in the maximum number of symbols. Apparently a gensize of 256 is too high for those devices, and that's why we routinely get execution times above 20 mins.
I used the test profile command-line option, which is already used in beem and imp to speed up the tests. The buildbot will just call the test binary with "--profile=embedded" on the slow builders. I think this is a flexible way to control some values in the tests (it is certainly better than using a define). And we don't sacrifice anything on the fast builders, we test with the same high gensizes on those!
We can also use the profiles if we want to fine-tune other things in the tests. Limiting the max gensize is just a first try.

I am very happy to report that a full kodo build can be completed in just 8 minutes (it was up to 26 minutes before the change). I think this is a must-have for quick development as it certainly improves the overall performance of the build system. No more waiting for those sluggish builders during a kodo build or a depcheck ;)

Some test execution times on the slow builders (BEFORE=>AFTER):
android23:  18 min => 4 min
raspberry: 21 min => 4.5 min
iPod Touch: 9 min => 3.5 min
openwrt_arm_32_vm: 8 min => 3.5 min