Makefile: delete output on error

I had a disk full condition and a partial hex file
got generated. Following make failed trying to use it.
We can make build a bit more robust by instructing
make to remove output files on error.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/Makefile b/Makefile
index 45ea48a..20b6dee 100644
--- a/Makefile
+++ b/Makefile
@@ -75,6 +75,7 @@
 
 # Make definitions
 .PHONY : all clean distclean FORCE
+.DELETE_ON_ERROR:
 
 vpath %.c src vgasrc
 vpath %.S src vgasrc