handle BOARD_KERNEL_BASE in releasetools

Some devices define a BOARD_KERNEL_BASE argument which must be given
as an argument to mkbootimg when building a bootable image.  Store the
value of this var (if any) in the target-files zip and use it when
building images.
diff --git a/core/Makefile b/core/Makefile
index 4089d4d..df41628 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -820,6 +820,9 @@
 ifdef BOARD_KERNEL_CMDLINE
 	$(hide) echo "$(BOARD_KERNEL_CMDLINE)" > $(zip_root)/RECOVERY/cmdline
 endif
+ifdef BOARD_KERNEL_BASE
+	$(hide) echo "$(BOARD_KERNEL_BASE)" > $(zip_root)/RECOVERY/base
+endif
 	@# Components of the boot image
 	$(hide) mkdir -p $(zip_root)/BOOT
 	$(hide) $(call package_files-copy-root, \
@@ -834,6 +837,9 @@
 ifdef BOARD_KERNEL_CMDLINE
 	$(hide) echo "$(BOARD_KERNEL_CMDLINE)" > $(zip_root)/BOOT/cmdline
 endif
+ifdef BOARD_KERNEL_BASE
+	$(hide) echo "$(BOARD_KERNEL_BASE)" > $(zip_root)/BOOT/base
+endif
 ifdef INSTALLED_RADIOIMAGE_TARGET
 	@# The radio image
 	$(hide) mkdir -p $(zip_root)/RADIO