constify default env

I can't see any obvious needs for the default environment to be writable,
so make it const.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/common/env_flash.c b/common/env_flash.c
index 456f2e8..50ca4ffa 100644
--- a/common/env_flash.c
+++ b/common/env_flash.c
@@ -74,7 +74,7 @@
 static ulong end_addr_new = CONFIG_ENV_ADDR_REDUND + CONFIG_ENV_SECT_SIZE - 1;
 #endif /* CONFIG_ENV_ADDR_REDUND */
 
-extern uchar default_environment[];
+extern const uchar default_environment[];
 
 
 uchar env_get_char_spec(int index)