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/woodburn_sd_defconfig b/configs/woodburn_sd_defconfig
index 0a5a8d5..96da4d8 100644
--- a/configs/woodburn_sd_defconfig
+++ b/configs/woodburn_sd_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_WOODBURN_SD=y
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
+CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/woodburn/imximage.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y