boot/bootutil: Make RAM loading methods public

RAM loading methods are currently private, which prevents external
modules with hooks from using them. This patch makes them public.

An interesting note is that a new method,
`boot_load_image_from_flash_to_sram()`, is added so that code can
actually set the image to be loaded - currently, that is inferred from
current boot loader state, but external code can't directly access
members of relevant `struct boot_loader_state`.

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