boards/x86: Adding clflush CPU capability to qemu_x86

Without this, cache manipulation cannot work in x86.
clflush was introduced with SSE2 extension, but may be implemented
without this extension and it seems that qemu made this choice a
adding sse2 CPU capability does not include clflush. It enabled by
default in 64 bits, so let's take care of this capability only for
the 32bits version of the board.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>