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/iconnect_defconfig b/configs/iconnect_defconfig
index eb0e8a9..53ba459 100644
--- a/configs/iconnect_defconfig
+++ b/configs/iconnect_defconfig
@@ -6,6 +6,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_IDENT_STRING=" Iomega iConnect"
 CONFIG_BOOTDELAY=3
+CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="iconnect => "
 # CONFIG_CMD_FLASH is not set