boot: zephyr: Disable HW stack protection

Some Zephyr boards enable HW stack protection by default, which
propagates in mcuboot build. Bootloader works fine with that option, but
the problem arises when we jump to application code. HW stack protection
catches application code during early init, which results in
"unresponsive" application.

Disable HW stack protection, so it doesn't make any harm for application
code.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>