Convert CONFIG_SPL_LIBGENERIC_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index 6299364..2696d86 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX31PDK=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_BOOTDELAY=1
 CONFIG_SPL=y
 CONFIG_CMD_BOOTZ=y