boot: zephyr: serial_recovery: Add boot mode enter ability

Adds an optional entrance method for mcuboot's serial recovery by
using Zephyr's boot mode retention system, this allows for an
application to set the retained data and reboot into the bootloader.
This also adds a selection of how to enter serial recovery mode, it
no longer requires having a GPIO entrance mechanism. Entrance
methods have been added under a new Kconfig menu.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>