include/inst: Store the instance root directory in a variable

include/inst: Store the instance root directory in a variable

The directory that all instance directories are contained within is
not kept in a variable, causing several functions to construct the
path from TOOLBOX_HOME. Hence, the root directory is effectively
declared in more than one place.

This commit introduces the global variable `__inst_root', which
points to the directory that contains all instance data, making it
unnecessary to construct the path from TOOLBOX_HOME.