zephyr: add an option to use GPIO to enable USB DFU.

Devices with a physical reset button might prefer to use it to enter USB
DFU mode, instead of always entering it with a timeout. Extract the
existing CONFIG_BOOT_SERIAL_DETECT detection code and use it to enter
DFU mode when CONFIG_BOOT_USB_DFU_GPIO is enabled.

This commit depends on zephyrproject-rtos/zephyr#30015, which changes
wait_for_usb_dfu from a nullary function that waits for a compile-time
fixed amount of time, to one that takes a timeout.

Signed-off-by: Josh Gao <josh@jmgao.dev>