sha1: add dedicated config option

The sha1 code is currently compiled for everyone, but in reality, it's
only used by the FIT code.  So make it optional just like MD5.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/lib_generic/Makefile b/lib_generic/Makefile
index c17179c..4b8ed3a 100644
--- a/lib_generic/Makefile
+++ b/lib_generic/Makefile
@@ -42,7 +42,7 @@
 COBJS-y += ldiv.o
 COBJS-$(CONFIG_MD5) += md5.o
 COBJS-y += net_utils.o
-COBJS-y += sha1.o
+COBJS-$(CONFIG_SHA1) += sha1.o
 COBJS-$(CONFIG_SHA256) += sha256.o
 COBJS-y += string.o
 COBJS-y	+= strmhz.o