unzip: use optimization from bitbake

Build with bitbake default optimizations, e.g. O2,
instead of custom O3. Reduces unzip binary package
size from 304401 to 283921 bytes, and enables building
with Os to even further reduce binary size if needed
and configured for the whole system.

Signed-off-by: Mikko Rapeli <mikko.rapeli@bmw.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>