samples: mgmt: hawkbit: force BOOT_MAX_IMG_SECTORS for stm32h573i_dk

The stm32h573i_dk target had the default value of BOOT_MAX_IMG_SECTORS
raised in efdd2a8f (west.yml: MCUboot synchronization from upstream).
This currently causes a build failure on some samples running with mcuboot
because the platform has not been moved to use external flash (which was
the reason for increasing BOOT_MAX_IMG_SECTORS)

To workaround this, force the value of BOOT_MAX_IMG_SECTORS for failing
tests.

Signed-off-by: Daniel DeGrasse <ddegrasse@tenstorrent.com>