This patch adds the following variables and functions:
$STARTDIR: the directory of the toplevel recipe
relative to the current recipe
$PARENTDIR: the directory of the parent recipe relative
to the current recipe
$CURDIR: `os.getcwd()`, just like in GNU make
$RECIPE: the basename of the current recipe file
startdir(), parentdir(): analgous to topdir() and
childdir(), but for $STARTDIR and $PARENTDIR
I also generalized path path handling in Utils.py with
more use of the os.path module.
I'm not sure if these work right in the presence of
":cd" commands, but then I'm not sure TOPDIR and
CHILDDIR work either.