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>