Update bootutil_sig_verify to receive slen as size_t

This fixes a comparison issue that was previously fixed by doing a
cast. Since tinycrypt and mbed-tls both already use an unsigned for
the size, the mcuboot interface was updated to also use an unsigned
value (size_t to be more precise!).

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index c1cf779..08e8c83 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -130,8 +130,8 @@
     uint8_t write_sz;
 };
 
-int bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, int slen,
-    uint8_t key_id);
+int bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig,
+                        size_t slen, uint8_t key_id);
 
 uint32_t boot_slots_trailer_sz(uint8_t min_write_sz);
 int boot_status_entries(const struct flash_area *fap);