image: use helper function for size units

image: use helper function for size units

Add the make function 'exp_units' for helping evaluate k/m/g size units in
expressions, and use this to consistently replace many ad hoc substitutions
like '$(subst k,* 1024,$(subst m, * 1024k,$(IMAGE_SIZE)))' in makefiles.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>