Add a struct for base, size in image.h

Quite a few things have a base address and a size, particularly in image.h

It might be better to have a 'struct range' like Linux:

struct range { ulong start; ulong end; };

Or we could use a different name and use start and size

Advantages:

• can pass a single arg to bootm_find_images() etc. instead of two
• can have generic function to check for overlap