lk: Fix issue with make not recompiling changes

Remove the target bootloader binary (appsboot.mbn) such
that source changes will trigger a recompile.
diff --git a/AndroidBoot.mk b/AndroidBoot.mk
index e5e2c28..772c248 100644
--- a/AndroidBoot.mk
+++ b/AndroidBoot.mk
@@ -3,10 +3,14 @@
 TARGET_BOOTLOADER := out/target/product/$(TARGET_PRODUCT)/appsboot.mbn
 BOOTLOADER_OUT := $(TOP)/$(TARGET_OUT_INTERMEDIATES)/BOOTLOADER_OBJ
 
+# Remove bootloader binary to trigger recompile when source changes
+appsbootldr_clean:
+	$(hide) rm -f $(TARGET_BOOTLOADER)
+
 $(BOOTLOADER_OUT):
 	mkdir -p $(BOOTLOADER_OUT)
 
-$(TARGET_BOOTLOADER): $(BOOTLOADER_OUT)
+$(TARGET_BOOTLOADER): $(BOOTLOADER_OUT) appsbootldr_clean
 	$(MAKE) -C bootable/bootloader/lk BOOTLOADER_OUT=../../../$(BOOTLOADER_OUT) $(TARGET_PRODUCT)