[DOCS] fast_model_boot.S: Pass the input-binary as IMAGE macro

So far we had fixed binary 'fast_model_boot.img' being included in the
Fast-Model boot code. This patch removes the limitation by having the binary
passed as macro IMAGE  (-DIMAGE=<input-binary-path> while compiling)

Signed-off-by: Sukanto Ghosh <sukantoghosh@gmail.com>