utils.bbclass: skip empty paths when handling FILESEXTRAPATHS

utils.bbclass: skip empty paths when handling FILESEXTRAPATHS

* if there is multiple .bbappend files with FILESEXTRAPATHS_prepend := "/:"
  then the one parsed last is causing trailing ':' and that's causing empty element in
  path = extrapaths.split(:) + path
* it's hard to keep all .bbappends from foreign layers to follow this rule, so it's better
  to be able to handle trailing ':'

(From OE-Core rev: 3b5591d423324da076d038ad335af47b616a7903)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>