kconfig: Don't USE_DT_CODE_PARTITION if there isn't one

Previously, if there was no `zephyr,code-partition` chosen DT node, then
enabling CONFIG_USE_DT_CODE_PARTITION would make all of flash available
to the linker, with no immediate indication of that to the user.

To avoid this discrepancy, make the USE_DT_CODE_PARTITION Kconfig symbol
depend on that chosen node being present. For the sake of verbosity, add
a dedicated symbol to describe this: HAS_DT_CODE_PARTITION.

An additional benefit is that certain platforms or applications can now
use `select USE_DT_CODE_PARTITION` to signal `zephyr,code-partition` as
a hard requirement, by failing the build whenever the node is missing.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>