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/socrates_defconfig b/configs/socrates_defconfig
index 1dadc12..58d135b 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -7,6 +7,8 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=1
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="echo;echo Welcome on the ABB Socrates Board;echo"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_BOARD_EARLY_INIT_R=y