kconfig: Convert device tree chosen properties to new kconfigfunctions

Convert how we get the various chosen properties like "zephyr,console"
to use the new kconfig functions like dt_chosen_to_label.

Because of how kconfig parses things we define a set of variables of the
form DT_CHOSEN_Z_<PROP> since comma's are parsed as field seperators in
macros.

This conversion allows us to remove code in gen_defines.py for the
following chosen properties:

zephyr,console
zephyr,shell-uart
zephyr,bt-uart
zephyr,uart-pipe
zephyr,bt-mon-uart
zephyr,uart-mcumgr
zephyr,bt-c2h-uart

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>