tools/env: use lib/crc32.c directly

Instead of linking the file into $(obj) tree use directly the source file.
This also prevents littered source tree if building not out-of-tree.

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/Makefile b/Makefile
index 7afd35b..d84b350 100644
--- a/Makefile
+++ b/Makefile
@@ -790,7 +790,7 @@
 	@rm -f $(obj)u-boot.ubl
 	@rm -f $(obj)u-boot.dtb
 	@rm -f $(obj)u-boot.sb
-	@rm -f $(obj)tools/{env/crc32.c,inca-swap-bytes}
+	@rm -f $(obj)tools/inca-swap-bytes
 	@rm -f $(obj)arch/powerpc/cpu/mpc824x/bedbug_603e.c
 	@rm -fr $(obj)include/asm/proc $(obj)include/asm/arch $(obj)include/asm
 	@rm -fr $(obj)include/generated