kconfig: Rename USE_CODE_PARTITION to USE_DT_CODE_PARTITION

USE_CODE_PARTITION is a bit vague as a symbol name ("use code partition
how?"). Rename it to USE_DT_CODE_PARTITION to make it clearer that it's
about devicetree.

This would break any third-party configuration files that set it, but
it'll generate an error since kconfig.py promotes warnings to errors, so
it's probably not a big deal.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>