linux-dtb.inc: Only append packages if not already in collection

* Fixes compatibility with the linux-yocto recipes

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
linux-yocto: Change SRCREV_externalcache to be static

* Select a specific SRCREV for the externalcache
* Resolves internet access requirement during parsing step

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>