We include newlib headers (and link part of newlib), so any
locally-defined function must have the same prototype as standard
ones. We'll soon nee a "puts" implementation for pp_printf, so this
adds puts as an alias of uart_write_string (at no size cost) and makes
uart_write_string itself return integer (this costs a few bytes).

While I'm at it, change uart_write_byte to be slightly faster.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>