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/netspace_v2_defconfig b/configs/netspace_v2_defconfig
index ffb60da..6427c79 100644
--- a/configs/netspace_v2_defconfig
+++ b/configs/netspace_v2_defconfig
@@ -9,6 +9,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200"
+CONFIG_USE_PREBOOT=y
 CONFIG_CONSOLE_MUX=y
 CONFIG_MISC_INIT_R=y
 # CONFIG_DISPLAY_BOARDINFO is not set