Prevent extra rebuild of fs.img by keeping intermediate object files.

Makefile

@@ -148,6 +148,12 @@ _forktest: forktest.o $(ULIB)
 mkfs: mkfs.c fs.h
 	gcc -Werror -Wall -o mkfs mkfs.c
 
+# Prevent deletion of intermediate files, e.g. cat.o, after first build, so
+# that disk image changes after first build are persistent until clean.  More
+# details:
+# http://www.gnu.org/software/make/manual/html_node/Chained-Rules.html
+.PRECIOUS: %.o
+
 UPROGS=\
 	_cat\
 	_echo\