acpi: add rules to compile ASL source

Detect presence of IASL compiler and use it
to process ASL source. If not there, use pre-compiled
files in-tree. Add script to update the in-tree files.

Note: distros are known to silently update iasl
so detect correct iasl flags for the installed version on each run as
opposed to at configure time.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

scripts/update-acpi.sh

@@ -0,0 +1,4 @@
+cd x86_64-softmmu
+for file in hw/i386/*.hex; do
+    cp -f $file ../$file.generated
+done