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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index e875626..94b9c78 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -15,6 +15,8 @@
 CONFIG_BOOTDELAY=-1
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="root=romfs"
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="echo U-BOOT for ${hostname};setenv preboot;echo"
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_DISPLAY_BOARDINFO=y
 CONFIG_SPL_BOARD_INIT=y