u-boot-xlnx: provide overrideable repo/branch plumbing
u-boot-xlnx: provide overrideable repo/branch plumbing

With these patches, top level component selection configuration can now
easily override u-boot-xlnx source repositories and branches.

Signed-off-by: Cyril Chemparathy <6d096e725578a07c9ac643febce9e1340b995155@xilinx.com>
Signed-off-by: Manjukumar Matha <4c64a17569570cb9f1048973bce031c402b55312@xilinx.com>

nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx