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/am335x_shc_sdboot_prompt_defconfig b/configs/am335x_shc_sdboot_prompt_defconfig
index 518703a..78d060c 100644
--- a/configs/am335x_shc_sdboot_prompt_defconfig
+++ b/configs/am335x_shc_sdboot_prompt_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SHC_SDBOOT=y
 CONFIG_SERIES=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000