generate_*_tests.py: chdir to mbedtls root

Do this in 2.28 just like it's done in the development branch, so that
code and command line usage that works on one branch doesn't surprisingly
fail on 2.28.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/mbedtls_dev/test_data_generation.py

@@ -29,6 +29,7 @@ import re
 from abc import ABCMeta, abstractmethod
 from typing import Callable, Dict, Iterable, Iterator, List, Type, TypeVar
 
+from mbedtls_dev import build_tree
 from mbedtls_dev import test_case
 
 T = TypeVar('T') #pylint: disable=invalid-name
@@ -182,6 +183,12 @@ def main(args, description: str, generator_class: Type[TestGenerator] = TestGene
                         help='List available targets and exit')
     parser.add_argument('targets', nargs='*', metavar='TARGET',
                         help='Target file to generate (default: all; "-": none)')
+
+    # Change to the mbedtls root, to keep things simple.
+    # Note that if any command line options refer to paths, they need to
+    # be adjusted first.
+    build_tree.chdir_to_root()
+
     options = parser.parse_args(args)
     generator = generator_class(options)
     if options.list: