semihosting: defer connect_chardevs a little more to use serialx

With that we can just use -semihosting-config chardev=serial0.

[AJB: tweak commit message]

Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <1592215252-26742-1-git-send-email-frederic.konrad@adacore.com>
Message-Id: <20200724064509.331-3-alex.bennee@linaro.org>