mcuboot: Kconfig options for single app slot RAM loading mode

MCUboot has a configuration for single application slot RAM loading, in
which the single loader (or a hook thereof) can load an application from
an arbitrary flash location to RAM. Applications that are to be loaded
in this way need to specify, in their mcuboot header, the load address
in RAM they are meant to be loaded.

This patch adds a new Kconfig for this mode. The load address used comes
from devicetree chosen property "mcuboot,ram-load-dev", if it exists,
and if not, "zephyr,sram".

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>