Normalize, test, and make constant $BuildRoot after loading scripts

[nightroman]

$BuildRoot is one of the special build variables. It tells to set this
directory current before invoking various build blocks, tasks in the first
place. By default, it is the directory of a build script. Unlike other special
variables, it can be changed by a script in order to use not default/standard
locations (e.g. when a script is in a subdirectory of the project directory).

Currently, IB does not do anything with potentially changed value. It may be a
missing directory, not a directory, or an invalid path. This may result in not
clear errors later. Or it may be a valid directory path but not normalized,
e.g. with \.. in it. The latter will work but this is not quite right.

TODO:

(1) After loading a build script IB should resolve and normalize the path of
$BuildRoot and test that the directory exists.

(2) After the above changes, it looks wise to make this variable constant in
order to protect it from accidental further changes. The build root may be
changed by the top level script code, once and for all.

[nightroman]

Custom build roots are especially useful when builds are defined by script
blocks, not as usual build scripts, see #78.

Unlike build scripts, blocks do not have own natural build roots because they
are not files in some directories. Their conventional build roots come from the
calling scripts. This is not always what builds require, so use of custom build
roots is especially important for script block builds.