x86-generic: add MMC support to boot off SD cards

MMC support has been added to x86-64 a while ago, there is no reason not
to support it in x86-generic as well.

Signed-off-by: Andreas Ziegler <github@andreas-ziegler.de>
[Matthias Schiffer: add commit message]
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

target/linux/x86/generic/config-default

@@ -205,6 +205,14 @@ CONFIG_MFD_INTEL_LPSS=y
 CONFIG_MFD_INTEL_LPSS_ACPI=y
 # CONFIG_MFD_INTEL_QUARK_I2C_GPIO is not set
 # CONFIG_MIXCOMWD is not set
+CONFIG_MMC=y
+CONFIG_MMC_BLOCK=y
+CONFIG_MMC_RICOH_MMC=y
+CONFIG_MMC_SDHCI=y
+CONFIG_MMC_SDHCI_PCI=y
+# CONFIG_MMC_SDHCI_PLTFM is not set
+# CONFIG_MMC_TIFM_SD is not set
+# CONFIG_MMC_WBSD is not set
 CONFIG_MMU_NOTIFIER=y
 # CONFIG_MOUSE_BCM5974 is not set
 # CONFIG_MOUSE_CYAPA is not set