boot/bootutil: Split RAM load code to its own file

RAM loading code is currently under bootutil/loader.c, and it's not
accessible for different loaders, such as the single loaders. Future
patches will make use of the RAM loading code outside the
bootutil/loader.c context, and this patch prepares for that by making it
standalone on boot/bootutil/src/ram_load.c

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