imximage: Encase majority of header in __ASSEMBLY__ declaration

Subsequent patches will want to include imageimage.h but in doing so
include it on an assembly compile path causing a range of compile errors.
Fix the errors pre-emptively by encasing the majority of the declarations
in imximage.h inside an ifdef __ASSEMBLY__ block.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Utkarsh Gupta <utkarsh.gupta@nxp.com>
Cc: Breno Lima <breno.lima@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>