zephyr: fix do_boot() warning on ARM
zephyr: fix do_boot() warning on ARM

Commit 7238f51 ("zephyr: add support for x86 booting method")
introduced a backend-specific do_boot() function. On ARM, this casts
to an undefined type 'struct vector_table*'.  Fix this by using the
'struct arm_vector_table*' introduced in that patch.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot