zephyr: Kconfig: change default for MCUBOOT_CLEANUP_ARM_CODE

Zephyr has introduced an option to perform the cleanup
of ARM core HW registers during early boot, when the
firmware is chain-loaded by MCUboot. Therefore, MCUboot
does not need to perform the same cleanup before jumping
to the application image. The patch relies on the fact
that building MCUboot with Zephyr implies loading also
a Zephyr-based application firmware. If this is not the
case, the application developer needs to enable the
MCUBOOT_CLEANUP_ARM_CODE Kconfig option manually, in the
project configuration.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>