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_nand.c b/common/env_nand.c
index 1b12c68..65a867d 100644
--- a/common/env_nand.c
+++ b/common/env_nand.c
@@ -59,8 +59,7 @@
 
 
 #if defined(ENV_IS_EMBEDDED)
-extern uchar environment[];
-env_t *env_ptr = (env_t *)(&environment[0]);
+env_t *env_ptr = &environment;
 #elif defined(CONFIG_NAND_ENV_DST)
 env_t *env_ptr = (env_t *)CONFIG_NAND_ENV_DST;
 #else /* ! ENV_IS_EMBEDDED */