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>

include/image.h

@@ -56,6 +56,7 @@
 #include <libfdt.h>
 #include <fdt_support.h>
 #define CONFIG_MD5		/* FIT images need MD5 support */
+#define CONFIG_SHA1		/* and SHA1 */
 #endif
 
 /*

lib_generic/Makefile

@@ -42,7 +42,7 @@ COBJS-y += lmb.o
 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