x86: coreboot: Show unimplemented sysinfo tags

Sometimes coreboot adds new tags that U-Boot does not know about. These
are silently ignored, but it is useful to at least know what we are
missing.

Add a way to collect this information. For Brya it shows:

   Unimpl. 38 41 37 34 42 40

These are:

   LB_TAG_PLATFORM_BLOB_VERSION
   LB_TAG_ACPI_CNVS
   LB_TAG_FMAP
   LB_TAG_VBOOT_WORKBUF
   LB_TAG_TYPE_C_INFO
   LB_TAG_BOARD_CONFIG

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>