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/mx6cuboxi_defconfig b/configs/mx6cuboxi_defconfig
index cc2ed9a..9fc443c 100644
--- a/configs/mx6cuboxi_defconfig
+++ b/configs/mx6cuboxi_defconfig
@@ -15,6 +15,8 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_BOOTCOMMAND="run findfdt; run finduuid; run distro_bootcmd"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="if hdmidet; then usb start; setenv stdin  serial,usbkbd; setenv stdout serial,vga; setenv stderr serial,vga; else setenv stdin  serial; setenv stdout serial; setenv stderr serial; fi;"
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL_TEXT_BASE=0x00908000