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>
b16e14a220