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/seaboard_defconfig b/configs/seaboard_defconfig
index 47c4540..efc8586 100644
--- a/configs/seaboard_defconfig
+++ b/configs/seaboard_defconfig
@@ -5,6 +5,8 @@
 CONFIG_TEGRA20=y
 CONFIG_TARGET_SEABOARD=y
 CONFIG_OF_SYSTEM_SETUP=y
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="usb start"
 CONFIG_SPL_TEXT_BASE=0x00108000
 CONFIG_SYS_PROMPT="Tegra20 (SeaBoard) # "
 # CONFIG_CMD_IMI is not set