cmake: app mem partion flexibility

This commit allows a more dynamic approach for specifying that a zephyr
library must be placed in a dedicated app memomory partition.
This is still done by defining the partition in code using
K_APPMEM_PARTITION_DEFINE, but now the zephyr library can be added to
the generation of partition table using zephyr_library_app_memory
instead of modifying the overall zephyr/CMakeLists.txt file.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>