linux-xlnx: Update SRC_URI to use https instead of git

* Cleaned up the comments for tags used

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
u-boot-xlnx: Update SRC_URI to use https instead of git

* Cleaned up the comments for tags used

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
machine-xilinx-default.inc: Specify virtual/bootloader

* Ensure that u-boot-xlnx is the default virtual/bootloader provider

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