[U-Boot] Patch to mkenvimage to handle text files with length that exceed env size

The current head revision of mkenvimage (e72be8947e129f5ab274c0a9f235d2cc0014b2ea) will prevent you from creating an env image from a text file that is larger than the env length specified by the '-s' option. That doesn't make sense given that the tool now allows comments and blank lines. This patch removes that limitation and allows longer text files to be used.

I don't have time / desire at the moment to figure out "patman" and could really care less if this is adopted up stream. Just figured I would share in case anybody else finds it useful enough to take time to do a proper patch.