bootutil: ecdsa P-256: Fix handling of sizes

The ECDSA signature is written as two DER-encoded INTEGERS.  Although
the values are always 256 bits, the encoding ends up being variable
length, because the encoding is signed, and therefore needs an extra
zero byte to keep the number positive.  This means that the length can
vary by up to two bytes.

The 'newt' tool handles this for signature by allowing space for the
largest encoding, and padding with one or two zeros.  However, the
bootutil image check code insists that the length is exact, resulting in
a decoding error on about 3/4 signatures.

Fix this by only verifying that we have at least enough payload to hold
the signature.  There are later checks that will fail if the integers
themselves are too large.

boot/bootutil/src/image_ec256.c

@@ -122,7 +122,7 @@ tinycrypt_decode_sig(uint32_t r[NUM_ECC_DIGITS], uint32_t s[NUM_ECC_DIGITS],
     if (rc) {
         return -1;
     }
-    if (cp + len != end) {
+    if (cp + len > end) {
         return -2;
     }
     rc = tinycrypt_read_bigint(r, &cp, end);