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/x600_defconfig b/configs/x600_defconfig
index b055434..a9604ad 100644
--- a/configs/x600_defconfig
+++ b/configs/x600_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_X600=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y