env: add prototypes for weak function

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>