blk: Hide the BLK and SPL_LEGACY_BLOCK options

We don't want boards to be able to change these. They can be handled
as dependencies of options that need them, such as SPL_MMC. There is no
point in enabling the block interface without any storage devices to
create a block device.

Hide both options from the 'menuconfig' display and deny their use in
defconfig files.

Signed-off-by: Simon Glass <sjg@chromium.org>