Convert CONFIG_SPL_LIBDISK_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/xilinx_zynqmp_zc1751_xm016_dc2_defconfig b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
index bdb6377..6b1d648 100644
--- a/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
+++ b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL=y
 # CONFIG_SPL_FAT_SUPPORT is not set
+# CONFIG_SPL_LIBDISK_SUPPORT is not set
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="ZynqMP> "