misc/printk: Make the default char_out routine weak

Architecture init code and/or HAL layers often have a working
"putchar" routine available long before the Zephyr driver layer is
initialized.

Make the default printk() output a weak symbol, so it can be
overridden on these platforms.

Also remove the kconfig depedency on CONSOLE_HAS_DRIVER, as this is a
non-driver mechanism.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>