global: Remove CONFIG_SYS_USE_DATAFLASH*

There are a handful of variants around CONFIG_SYS_USE_DATAFLASH and none
of them now control anything further within their board config.h files,
so remove these from CONFIG_SYS_EXTRA_OPTIONS and then remove the empty
blocks in the board config.h files.  In a few places further clean up
related logic.

Signed-off-by: Tom Rini <trini@konsulko.com>