fix problems with new image size calculations

The build servers have GNU coreutils 5.93, where stat does not output
a newline.  Ubuntu hardy has GNU coreutils 6.10, where it does.
Lacking a newline messes up the summing of the sizes.  Fix
get-file-size to remove the newline if present, and make the total
calculation in assert-max-file-size more robust.

Also, if the image was too big, it was not actually making the build
fail (because /bin/false was not the last thing called).  Fix that so
it does.