Add "Apply update from sdcard" to default recovery image.

At the moment, this is the only difference in the sprout recovery
image. That's silly. Let's just improve the error handling slightly
and always have this option present.

Also make the obscure "<3e>" less unclear.

Also use "power button" as the default text rather than "enter button",
because it's been years since anyone had one of those. (Longer term we
should let subclassers tell us the keycode and we translate it to the
correct string.)

Also move the two "Reboot" options together, put "Power off" at the
bottom (and use that terminology, like the real UI, rather than
"Power down"), and use capitals throughout.

Finally, add a README.md with some useful instructions.

Change-Id: I94fb19f73d79c54fed2dda30cefb884426641b5c