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>

src/Kconfig

@@ -1024,6 +1024,24 @@ config DEBUG_ADA_CODE
 	  Add the compiler switch `-gnata` to compile code guarded by
 	  `pragma Debug`.
 
+config HAVE_EM100_SUPPORT
+	bool "Platform can support the Dediprog EM100 SPI emulator"
+	help
+	  This is enabled by platforms which can support using the EM100.
+
+config EM100
+	bool "Configure image for EM100 usage"
+	depends on HAVE_EM100_SUPPORT
+	help
+	  The Dediprog EM100 SPI emulator allows fast loading of new SPI images
+	  over USB. However it only supports a maximum SPI clock of 20MHz and
+	  single data output. Enable this option to use a 20MHz SPI clock and
+	  disable "Dual Output Fast Read" Support.
+
+	  On AMD platforms this changes the SPI speed at run-time if the
+	  mainboard code supports this. On supported Intel platforms this works
+	  by changing the settings in the descriptor.bin file.
+
 endmenu