scons, python: Remove SmartDict from python utilities

scons, python: Remove SmartDict from python utilities

The SmartDict, used by buildEnv, has been added long time ago for
the following reasons: (checking its documentation)

---
The SmartDict class fixes a couple of issues with using the content
of os.environ or similar dicts of strings as Python variables:

1) Undefined variables should return False rather than raising KeyError.

2) String values of 'False', '0', etc., should evaluate to False
   (not just the empty string).
---

These are valid reasons, but I believe they should be addressed in
a more standardized way by using a common dictionary.

1) We should simply rely on dict.get

if buildEnv.get('KEY', False/None):

2) We should discourage the use of stringified False or 0.
If we are using a dictionary, can't we just pass those values as
booleans?
The SmartDict is basically converting every value into a
string ("Variable") at every access (__getitem__)
The Variable is a string + some "basic" conversion methods
What is the problem of passing every dict value as a string?

The problem is the ambiguity on the boolean conversion.

If a variable is modelling a boolean, we can return true if
the value is 'yes', 'true'... and false if the value is
'no', 'false' etc. We should raise an exception if it is
something different, like a typo (e.g.) 'Fasle'.
But if the variable is not modelling a boolean, we don't know
how to handle that. How should we convert 'mystring' ?

If we decide to treat 'mystring' as True (which is basically
what a str.__bool__ would return) we will break typoes detection,
as 'Fasle' will now be converted to True, rather than raising
an exception.

Change-Id: I960fbfb1ec0f703e1e372dd752ee75f00632acac
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37775
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>