mcumgr/img_mgmt: Fix zcbor logic in os_mgmt_bootloader_info

There have been to problems with the code where zcbor_bool_encode
has been fed value instead of expected pointer and the result
of previous zcbor_encode operations has not been taken to evaluate
value of ok status.
The change also replaces usage of  #if IS_ENABLED with #ifdef, as
IS_ENABLED should not be used outside if().

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>