stmhal: Add Python function to set UART for REPL.

This adds a hook to get/set pyb_uart_global_debug from Python, using
pyb.repl_uart().  You can set it to an arbitrary UART object, and then
the REPL (in and out) is repeated on this UART object (as well as on USB
CDC).

Ultimately, this will be replaced with a proper Pythonic interface to
set sys.stdin and sys.stdout.