Added external repo fpgahdl_xilinx from analog devices.

Added helper scripts for external repo fetch & sparse checkout.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "fpga/externals/fpgahdl_xilinx"]
+	path = fpga/externals/fpgahdl_xilinx
+	url = https://github.com/analogdevicesinc/fpgahdl_xilinx

fpga/externals/README.md

@@ -6,4 +6,26 @@ This directory is to hold checkouts of repositories from external sources that a
 
 - 1  ADI HDMI IP:  https://github.com/analogdevicesinc/fpgahdl_xilinx
 
+## Fetching content of external repos:
+
+When this repo is cloned, there will be a directory for each external repository but initially it will be empty.
+To populate it when needed, run the get_xxxx script for that repository, e.g. ./get_fpgahdl_xilinx.
+
+## Adding external repos:
+
+```
+$ cd <root of local repository>
+$ git submodule add <repository URL> fpga/externals/<name of repo>
+
+# For example:
+
+$ cd ../..
+$ git submodule add https://github.com/analogdevicesinc/fpgahdl_xilinx fpga/externals/fpgahdl_xilinx
+```
+Then add a corresponding get_xxxx script and optionally a list of directories to include if doing a sparse checkout.
+Add those files to git, and commit the new setup.
+
+# NOTES
+
+Do not make any changes to the external repository's files or folders from here, submodules are notoriously difficult to deal with if changes are made locally.
 

fpga/externals/fpgahdl_xilinx.sco

@@ -0,0 +1,4 @@
+/cf_lib
+/README.md
+/.git*
+

fpga/externals/get_fpgahdl_xilinx

@@ -0,0 +1,10 @@
+pushd ../../
+git submodule init
+git submodule update
+popd
+cp fpgahdl_xilinx.sco ../../.git/modules/fpga/externals/fpgahdl_xilinx/info/sparse-checkout
+cd fpgahdl_xilinx
+git config core.sparsecheckout true
+git read-tree -mu HEAD
+cd ..
+