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/rpi_0_w_defconfig b/configs/rpi_0_w_defconfig
index 1ab35f1..3d56b4a 100644
--- a/configs/rpi_0_w_defconfig
+++ b/configs/rpi_0_w_defconfig
@@ -5,6 +5,8 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="usb start"
 CONFIG_MISC_INIT_R=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set