This make MCUboot build as Zephyr application.
Providing optinal 2nd stage bootloader to the
IDF bootloader, which is used by default.
This provides more flexibility when building
and loading multiple images and aims to
brings better DX to users by using the sysbuild.
MCUboot and applications has now separate
linker scripts.
Signed-off-by: Marek Matej <marek.matej@espressif.com>