env: allow people to force envcrc building

For people who want to manually extract the embedded environment so that
it can be manually packed into the final u-boot image, add a config opt
to force building of the envcrc tool.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/README b/README
index 2cad44d..cb08816 100644
--- a/README
+++ b/README
@@ -2563,6 +2563,11 @@
 of environment data (variable area); in general, we support the
 following configurations:
 
+- CONFIG_BUILD_ENVCRC:
+
+	Builds up envcrc with the target environment so that external utils
+	may easily extract it and embed it in final U-Boot images.
+
 - CONFIG_ENV_IS_IN_FLASH:
 
 	Define this if the environment is in flash memory.