Add handling of standard "non-removable" property, and also some workaround
code so that if card detect is wired to a gpio pin, for now we just treat
it the same as non-removable (because there isn't a gpio driver yet).
Enable both sdcard slots, but not the sdio-based wifi that we don't yet
have a driver for.
Validate the header of a new dtb before using it.  Remove the comment
that says that should be done.