Search for scripts relative to the executable on all(?) platforms

Add a helper to hide the platform-dependent method to get a
canonical, absolute, /-separated path to the executable.

Use this and the relative path from BINDIR to PKGDATADIR to
construct a search path that finds the scripts even if the
installation dir is moved, as long as the structure below $prefix
is maintained.

This method should fully support all the tricks you can to with
autotools to customize the installed layout such as overriding the
default directories at configure-time and overriding the configured
directories at build-time.

The exe path detection methods are combined from
http://openocd.zylin.com/3388 by Rick Foos and
http://openocd.zylin.com/3537

 by Steven Stallion, as well as tips
found all over internet.

Change-Id: Ifc9cc9dd0bf52fbd67b1b0f2383318cda0c422c4
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Signed-off-by: Steven Stallion <sstallion@gmail.com>
Reviewed-on: http://openocd.zylin.com/3889


Tested-by: jenkins
Reviewed-by: Rick Foos <rfoos@solengtech.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>