external/opal-prd: Make compilation prerequisites as order-only prerequisites
external/opal-prd: Make compilation prerequisites as order-only prerequisites

Makefile creates symbolic link to libflash, ccan, etc. Presently we added them
as normal prerequisites. In some cases (like we compile skiboot after compiling
opal-prd) it will result in recompiling source again during make install. Hence
make it as order-only prerequisites.

Signed-off-by: Vasant Hegde <a6d753843573f7bfed5075c1e875b0be8007a0ab@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

legoater/skiboot,mikey/skiboot,open-power/skiboot,shenki/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,mikey/skiboot,open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,legoater/skiboot,mikey/skiboot,stewart-ibm/skiboot