env: add prototypes for weak function

This patch adds prototypes for several weak functions:
- env_ext4_get_intf
- env_ext4_get_dev_part
- env_get_location

It solves the following warnings when compiling with W=1
on stm32mp1 board:

board/st/stm32mp1/stm32mp1.c:849:19: warning: no previous prototype for 'env_get_location' [-Wmissing-prototypes]
 enum env_location env_get_location(enum env_operation op, int prio)
                   ^~~~~~~~~~~~~~~~
board/st/stm32mp1/stm32mp1.c:876:13: warning: no previous prototype for 'env_ext4_get_intf' [-Wmissing-prototypes]
 const char *env_ext4_get_intf(void)
             ^~~~~~~~~~~~~~~~~
board/st/stm32mp1/stm32mp1.c:889:13: warning: no previous prototype for 'env_ext4_get_dev_part' [-Wmissing-prototypes]
 const char *env_ext4_get_dev_part(void)
             ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>

include/env_internal.h

@@ -211,6 +211,26 @@ struct env_driver {
 
 extern struct hsearch_data env_htab;
 
+/**
+ * env_ext4_get_intf() - Provide the interface for env in EXT4
+ *
+ * It is a weak function allowing board to overidde the default interface for
+ * U-Boot env in EXT4: CONFIG_ENV_EXT4_INTERFACE
+ *
+ * @return string of interface, empty if not supported
+ */
+const char *env_ext4_get_intf(void);
+
+/**
+ * env_ext4_get_dev_part() - Provide the device and partition for env in EXT4
+ *
+ * It is a weak function allowing board to overidde the default device and
+ * partition used for U-Boot env in EXT4: CONFIG_ENV_EXT4_DEVICE_AND_PART
+ *
+ * @return string of device and partition
+ */
+const char *env_ext4_get_dev_part(void);
+
 /**
  * env_get_location()- Provide the best location for the U-Boot environment
  *