lmb: Enable LMB if SYS_BOOT_RAMDISK_HIGH

Ramdisk relocation requires LMB, so enable it automatically to avoid
build errors.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index 170ab63..8fa3181 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -814,6 +814,7 @@
 	depends on CMD_BOOTM || CMD_BOOTI || CMD_BOOTZ
 	depends on !(NIOS2 || SANDBOX || SH || XTENSA)
 	def_bool y
+	select LMB
 	help
 	  Enable initrd_high functionality.  If defined then the initrd_high
 	  feature is enabled and the boot* ramdisk subcommand is enabled.