Update rc.firmware

Previous USB patch to add 4MB to partition size calculation was not only not working, but worse, causing dev's to close all related tickets as duplicates when it clearly still needed fixing.

Also, some dev's cited that "the CF install base is so much larger it's not worth breaking". Well, you've already broken it by adding a 4MB buffer for USB, which doesn't work. Either patch it right at 6MB, or completely take out the buffer all together to "fix" CF upgrades again. Why sit on a double broken patch?

PoC: EVERY USB flash based install in place upgrade was failing by exactly 2MB. Added this patch to my 2.0.2 system after a failed 2.0.3 upgrade and boom, upgraded to 2.0.3 without issue.

You're welcome ;-)
Merge pull request #590 from ky41083/patch-1

Update rc.firmware again to support upgrades on USB Flash devices that are off by even larger amounts.