BUILD: makefile: move -O2 from CPU_CFLAGS to OPT_CFLAGS

BUILD: makefile: move -O2 from CPU_CFLAGS to OPT_CFLAGS

CPU_CFLAGS is meant to set the CPU-specific options (-mcpu, -march etc).
The fact that it also includes the optimization level is annoying because
one cannot be set without replacing the other. Let's move the optimization
level to a new independent OPT_CFLAGS that is added early to the list, so
that other CFLAGS (including CPU_CFLAGS) can continue to override it if
necessary.