env: move more common code to env_import_redund

There is more common code in mmc, nand and ubi env drivers that
can be shared by moving to env_import_redund.

For this, a status/error value whether the buffers were loaded
are passed as additional parameters to env_import_redund.
Ideally, these are already returned to the env driver by the
storage driver. This is the case for mmc, nand and ubi, so for
this change, code deduplicated.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>

include/environment.h

@@ -297,7 +297,8 @@ int env_export(env_t *env_out);
 
 #ifdef CONFIG_SYS_REDUNDAND_ENVIRONMENT
 /* Select and import one of two redundant environments */
-int env_import_redund(const char *buf1, const char *buf2);
+int env_import_redund(const char *buf1, int buf1_status,
+		      const char *buf2, int buf2_status);
 #endif
 
 /**