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/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index fe2ecf6..5cd9601 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -5,6 +5,8 @@
 CONFIG_TARGET_NOKIA_RX51=y
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_BOOTDELAY=30
+CONFIG_USE_PREBOOT=y
+CONFIG_PREBOOT="setenv mmcnum 1; setenv mmcpart 1;setenv mmcscriptfile bootmenu.scr;if run switchmmc; then setenv mmcdone true;setenv mmctype fat;if run scriptload; then true; else setenv mmctype ext2;if run scriptload; then true; else setenv mmctype ext4;if run scriptload; then true; else setenv mmcdone false;fi;fi;fi;if ${mmcdone}; then run scriptboot;fi;fi;if run slide; then true; else setenv bootmenu_delay 0;setenv bootdelay 0;fi"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_HUSH_PARSER=y