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/MPC8349EMDS_SLAVE_defconfig b/configs/MPC8349EMDS_SLAVE_defconfig
index 3a1268a..7550aa3 100644
--- a/configs/MPC8349EMDS_SLAVE_defconfig
+++ b/configs/MPC8349EMDS_SLAVE_defconfig
@@ -84,6 +84,8 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="PCISLAVE"
 CONFIG_BOOTDELAY=6
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="echo;echo Type \"run flash_nfs\" to mount root filesystem over NFS;echo"
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_I2C=y