autoupdate is broken

[gtxaspec]

changes made by the upstream maintainers broke autoupdate.
In it's current form it doesn't work with any files, corrupts the overlay partition, and slows down the boot process if no sd card is present.

1. complete refactor of cmd_sdupdate
2. possibly combine with cmd_sdstart

[gtxaspec]

full image should write the flash, then either rename or delete, to prevent an update loop as it currently does.

[gtxaspec]

our u-boot version's FAT command doesn't support rename or delete, so lets check before flashing for autoupdate-full.flag before flashing, if its not present, proceed. after flashing, create the flag file. erase in OS from init.d script after confirming successful flash.

[gtxaspec]

full image fixed in 85d5d61

[gtxaspec]

We should read all files on SD card, index them, check the index for the autoupdate and kernel files, instead of checking for each file, one at a time, in each function.