boot: zephyr: move to non-deprecated GPIO flags

GPIO_DIR_IN has been replaced by GPIO_INPUT, GPIO_PUD_PULL_UP by
GPIO_PULL_UP, and gpio_pin_read() by gpio_pin_get_raw().  Update the
code to use the preferred API if it available.  This avoids
deprecation warnings in the build.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>