x86: smbios: Add a Kconfig indicating SMBIOS-table presence

When booted from coreboot, U-Boot does not build the SMBIOS tables, but
it should still pass them on to the OS. Add a new option which indicates
whether SMBIOS tables are present, however they were built.

Flip the ordering so that the dependency is listed first, which is less
confusing.

Adjust GENERATE_SMBIOS_TABLE to depend on this new symbol.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>