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/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 002db24..6f4fe45 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -18,6 +18,8 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_BOOTCOMMAND="run findfdt; run distro_bootcmd; run legacy_bootcmd"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="usb start;sf probe"
 CONFIG_MISC_INIT_R=y
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_SPL_TEXT_BASE=0x00908000