boot_serial: Fix build for 64-bit architectures

Typecasting pointer variables to uint32_t
instead of uintptr_t was causing build error
on 64-bit architecture.

This is useful, because I am currently working
on implementing support for 64-bit native target
in Apache Mynewt. There is unit test for boot_serial
and it cannot be compiled without this changes.

Signed-off-by: Michal Gorecki <michal.gorecki@codecoup.pl>

boot/boot_serial/src/boot_serial.c

@@ -1037,7 +1037,7 @@ boot_serial_output(void)
  char encoded_buf[BASE64_ENCODE_SIZE(sizeof(buf))];
 
  data = bs_obuf;
- len = (uint32_t)cbor_state->payload_mut - (uint32_t)bs_obuf;
+ len = (uintptr_t)cbor_state->payload_mut - (uintptr_t)bs_obuf;
 
  bs_hdr->nh_op++;
  bs_hdr->nh_flags = 0;