west: runners: stm32flash: convert start_addr to string (concatenation)

The start_addr attribute of Stm32flashBinaryRunner is an integer. It
must be converted to a string before being concatenated with a colon and
the (already converted to a string) size to erase or write.

Signed-off-by: Grzegorz Szymaszek <gszymaszek@short.pl>