boot: zephyr: cmake: Fix issue with missing dts entries

Fixes an issue whereby a device might not have a write or erase
entry for the flash controller in devicetree. In the case whereby
the other slot has this information, use that instead. In the case
whereby neither slot has this information, use default values and
show a warning to the user

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