bootutil: Remove duplicated code from bootutil_misc.c

Duplicated functions, existing in bootutil_public.c have been removed
from bootutil_misc.c.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/bootutil_public.c b/boot/bootutil/src/bootutil_public.c
index 5afe305..7330fc9 100644
--- a/boot/bootutil/src/bootutil_public.c
+++ b/boot/bootutil/src/bootutil_public.c
@@ -334,7 +334,7 @@
  *
  * @returns 0 on success, != 0 on error.
  */
-static int
+int
 boot_write_trailer(const struct flash_area *fap, uint32_t off,
         const uint8_t *inbuf, uint8_t inlen)
 {
@@ -362,7 +362,7 @@
     return 0;
 }
 
-static int
+int
 boot_write_trailer_flag(const struct flash_area *fap, uint32_t off,
         uint8_t flag_val)
 {