stmhal: Set entry point for ELF binary debugging

When loading the ELF binary to the board with a debugger, the debugger
needs to know at which point to start executing the code. Currently the
entry point defaults to the start of the .text section.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Implement kwargs for builtin open() and _io.FileIO

This makes open() and _io.FileIO() more CPython compliant.
The mode kwarg is fully iplemented.
The encoding kwarg is allowed but not implemented; mainly to allow
the tests to specify encoding for CPython, see #874
Use mode/encoding kwargs in io and unicode tests

mode argument is used to assert it works
encoding argument is used to make sure CPython uses the correct encoding
as it does not automatically use utf-8
Merge pull request #922 from swegener/for-upstream

stmhal: Set entry point for ELF binary debugging