uboot-lantiq: mark SPL variants as broken

All SPL variants are lzo compressed. The lzop binary is used for
compression but is not available in tools.

Additionally at least the NAND SPL support is broken and doesn't create
working bootloaders.

The fb3370 SPI NOR SPL enabled u-boot isn't required for LEDE since the
LEDE images are targeting the pre-installed EVA bootloader.

Mark these u-boot variants as well as the SPL variants for the
reference boards as broken till the lzma issues are fixed upstream and
we can use lzma instead of lzo compression.

Signed-off-by: Mathias Kresin <dev@kresin.me>