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/bg0900_defconfig b/configs/bg0900_defconfig
index c5e59a5..bdf29d6 100644
--- a/configs/bg0900_defconfig
+++ b/configs/bg0900_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_BG0900=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y