relocatable.bbclass: Enhancements to rpath replacement algorithm
This patch removes the hard coded number of parent directory operators ( /..)
placed into the rpath and instead fully dynamically generates the rpath entries
based on the current rpath.
Theoretically this patch means we can now make cross packages relocatable but
this is *not* enabled as chrpath can only set a new rpath of the same length
or shorter than the existing rpath. Due to the amount of parent directory
jumps we need to encode in the rpath this can easily fail when TMPDIR is in a
short namespace (e.g. /usr/poky).
