Current pxe init script does not support pxe for centos distro as
base image. Different folders were checked for centos to copy
pxe/ipxe files to tftpboot folder.
Change-Id: I4911825193d75aaaed24e8b71ba43efa2fc78fe8
Signed-off-by: Mingyuan Qi <mingyuan.qi@intel.com>