pc-bios/s390-ccw: Optimize the s390-netboot.img for size

pc-bios/s390-ccw: Optimize the s390-netboot.img for size

The -O2 optimization flag is passed via CFLAGS to the firmware Makefile,
but in netbook.mak, we've got some rules that only use QEMU_CFLAGS for
compiling the libc and libnet from SLOF, so these files get compiled
without optimization so far. Use CFLAGS here, too, to create faster
and smaller code.

We can additionally save some more bytes in the firmware images by compi-
ling the code with -fno-asynchronous-unwind-tables. This will omit some
ELF sections (used for stack unwinding for example) from the image that
we do not need in the firmware.

Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>