doc: release: 3.4: Add not on Kconfig mgmt changes

Adds a note that FLASH_MAP and STREAM_FLASH now also need to be
selected to be able to enable the mcuboot image manager Kconfig
option, to avoid a cmake dependency loop, and Kconfigs required
for other mgmt subsystems.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>