Convert CONFIG_USE_PREBOOT and CONFIG_PREBOOT to Kconfig

This converts the following to Kconfig:
   CONFIG_USE_PREBOOT
   CONFIG_PREBOOT

Both are together in one commit, since otherwise the former causes kconfig
to define the latter, which gives duplicate symbol errors.

Includes a manual fixup for CONFIG_PREBOOT in ids8313_defconfig since the
backslash lands in the wrong place. Similarly with socfpga_vining_fpga.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/syzygy_hub_defconfig b/configs/syzygy_hub_defconfig
index ac1bac8..8b72983 100644
--- a/configs/syzygy_hub_defconfig
+++ b/configs/syzygy_hub_defconfig
@@ -16,6 +16,7 @@
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_LEGACY_IMAGE_FORMAT=y
+CONFIG_USE_PREBOOT=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_PROMPT="Zynq> "