default path for PREBUILT common images

[pooyaww]

The default path for Alveo accelerator card base platforms is /opt/xilinx/platform(s)/ while the make file in this repo considers /opt/xilinx/platform/ as the default path for the prebuilt common image. While the purpose of these two folders are different , It can easily make confusion. I think choosing a different default folder name or even selecting platform(s) as common parent folder can omit this naming confusion specially for those who work with both PCIe accelerator cards and embedded platforms.