get.sh: put tmpclone dirs in xbtmp

and generate them, don't hardcode them - this reduces
the chance of race conditions, which we have seen in
the past and which current execution flow in xbmk even
mitigates in a few places, by doing things in a certain
order.

this change makes the code more robust and easier to
maintain.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/tree.sh

@@ -65,8 +65,6 @@ trees()
 	targets="$*"
 	cmd="build_targets $targets"
 	singletree "$project" && cmd="build_project"
-
-	remkdir "${tmpgit%/*}"
 }
 
 build_project()