env: move extern environment[] to environment.h

Extract all extern declarations for environment out of c files
into the environment.h header.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/common/env_flash.c b/common/env_flash.c
index d9f8fcb..e770ee6 100644
--- a/common/env_flash.c
+++ b/common/env_flash.c
@@ -49,9 +49,7 @@
 char * env_name_spec = "Flash";
 
 #ifdef ENV_IS_EMBEDDED
-
-extern uchar environment[];
-env_t *env_ptr = (env_t *)(&environment[0]);
+env_t *env_ptr = &environment;
 
 static env_t *flash_addr = (env_t *)CONFIG_ENV_ADDR;