Kconfig: Make the EM100 config option common

This applied to AMD devices as well as Intel, although the mechanism is
different. Move the option to a common place.

BUG=b:111363976
TEST=USE=em100-mode emerge-reef coreboot
See that a message appears:
 * Enabling em100 mode (slow SPI flash)

Change-Id: Iea437bdf42e7bc49b1d28c812bfc6128e3eb68bd
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://review.coreboot.org/27467
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
diff --git a/src/southbridge/intel/common/firmware/Kconfig b/src/southbridge/intel/common/firmware/Kconfig
index 30c9aaf..c9cf6f8 100644
--- a/src/southbridge/intel/common/firmware/Kconfig
+++ b/src/southbridge/intel/common/firmware/Kconfig
@@ -26,6 +26,7 @@
 
 config HAVE_IFD_BIN
 	bool "Add Intel descriptor.bin file"
+	select HAVE_EM100_SUPPORT  # We use ifdtool to enable this.
 	help
 	  The descriptor binary
 
@@ -34,12 +35,6 @@
 	default "3rdparty/blobs/mainboard/$(MAINBOARDDIR)/descriptor.bin"
 	depends on HAVE_IFD_BIN && !BUILD_WITH_FAKE_IFD
 
-config EM100
-	bool "Configure IFD for EM100 usage"
-	depends on HAVE_IFD_BIN && !BUILD_WITH_FAKE_IFD
-	help
-	  Set SPI frequency to 20MHz and disable Dual Output Fast Read Support
-
 config HAVE_ME_BIN
 	bool "Add Intel ME/TXE firmware"
 	depends on HAVE_IFD_BIN