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 <marti.bolivar@linaro.org>