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/sama5d4ek_nandflash_defconfig b/configs/sama5d4ek_nandflash_defconfig
index 2ec8dde..a8d503f 100644
--- a/configs/sama5d4ek_nandflash_defconfig
+++ b/configs/sama5d4ek_nandflash_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_SAMA5D4EK=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D4,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3