configs: enable additional compiler warnings

configs: enable additional compiler warnings

Make it easier to spot configuration problems like commit 886ca63f.
gcc -Wextra includes -Woverride-init, which will print warnings like:

qemu-vm.c:190:3: warning: initialized field overwritten [-Woverride-init]
   [  0x78/8 ...   0x7f/7] = -1,
   [  0x80/8 ...   0x87/8] = 0xfe, /* Linux: native_io_delay() */
   ^
qemu-vm.c:190:3: warning: (near initialization for 'config.pio_bitmap[16]')

see also: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

Signed-off-by: Georg Schiesser <georg.schiesser@opentech.at>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>