debootstrap: Use relative pathnames in sha256sum

Prior to this, debootstrap sha256sums would contain the full path to the
filename passed to the tool upon generation, which would break for anyone but
the originator of the tarball.

Change-Id: I6e9ce86e94eac1c6168c130f6498372cd13b8163
diff --git a/debootstrap.mk b/debootstrap.mk
index 8d0a69c..d4a5cb2 100644
--- a/debootstrap.mk
+++ b/debootstrap.mk
@@ -26,7 +26,7 @@
 
 make-bootstrap-sha256sum: $(DEBOOTSTRAP_TARBALL)
 	cd $(ROOTDIR)/cache && \
-		sha256sum $(DEBOOTSTRAP_TARBALL) > $(DEBOOTSTRAP_TARBALL_SHA256)
+		sha256sum $(notdir $(DEBOOTSTRAP_TARBALL)) > $(DEBOOTSTRAP_TARBALL_SHA256)
 
 make-bootstrap-tarball: $(ROOTDIR)/build/debootstrap.mk
 	mkdir -p $(PRODUCT_OUT)/obj/DEBOOTSTRAP