src/*/intel/: clarify Kconfig options regarding IFD

HAVE_INTEL_FIRMWARE is used to enable certain options that rely on a valid
Inter Flash Descriptor to exist. It does *not* identify platforms or boards
that are capable of running in descriptor mode if it's valid.
Refine the help text to make this clear.

Introduce a new option INTEL_DESCRIPTOR_MODE_CAPABLE that does simply
declare that IFD is supported by the platform. Select this value everywhere
instead of the HAVE_INTEL_FIRMWARE and default HAVE_INTEL_FIRMWARE to
y if INTEL_DESCRIPTOR_MODE_CAPABLE is selected.

Move the QEMU Q35 special case (deselection of HAVE_INTEL_FIRMWARE) to
the mainboard directory.

Change-Id: I4791fce03982bf0443bf0b8e26d9f4f06c6f2060
Signed-off-by: Stefan Tauner <stefan.tauner@gmx.at>
Reviewed-on: https://review.coreboot.org/28371
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/southbridge/intel/ibexpeak/Kconfig

@@ -35,7 +35,7 @@ config SOUTH_BRIDGE_OPTIONS # dummy
 	select HAVE_USBDEBUG_OPTIONS
 	select COMMON_FADT
 	select ACPI_SATA_GENERATOR
-	select HAVE_INTEL_FIRMWARE
+	select INTEL_DESCRIPTOR_MODE_CAPABLE
 	select SOUTHBRIDGE_INTEL_COMMON_GPIO
 	select HAVE_INTEL_CHIPSET_LOCKDOWN