conf: make release build use -O3

In some workloads it noticably improves performance. I measured
6% increase in netperf throughput on my laptop.

Object file size is only slightly bloated:

 loader.elf (O2): 47246227
 loader.elf (O3): 51272625 (+8.5%)

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>