sandbox: Remove OF_HOSTFILE

OF_HOSTFILE is used on sandbox configs only.  Although it's pretty
unique and rarely causes confusion,  we are better of having simpler
config options for the DTB.

So let's replace that with the existing OF_BOARD.  U-Boot would then
have only three config options for the DTB origin.
- OF_SEPARATE, build separately from U-Boot
- OF_BOARD, board specific way of providing the DTB
- OF_EMBED embedded in the u-boot binary(should not be used in production)

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>