external-xilinx-toolchain: Fix Parsing error

* Fix Version Parsing error

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
linux-machine-config.inc: Refactored useful python functions

* Refactored the useful python functions into more generic form and
  placed them in the 'xilinx-utils.bbclass' which can be inherited for
  the functions.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
u-boot-extra.inc: Refactored to use xilinx-utils functions

* Fixed issue with manual searching of BBPATH
* Cleaned up to use same SRC_URI append method as linux-machine-config

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