devices: implementation details in Kit management

The list of installed devices and their associated locations is kept in c:\Program Files\Common Files\Symbian\devices.xml. When a kit is installed, the installer program updates this file appropriately.

A kit installs a small stub program for each tool in c:\Program Files\Common Files\Symbian\Tools\. This location is put first in the path so that the stub rather than the real tool is called.

When a tool stub is run, it:

determines the device using the default settings and any specified overrides
sets an environment variable EPOCROOT to the path of the device's epoc32\ tree (not including the drive letter). Real tools use this variable when they need to work out a full path.
modifies the path environment variable, to allow a tool to subsequently call further tools from the same tools location
invokes the real tool

Original settings of both the path and the EPOCROOT are preserved after the stub (and the real tool) returns.