dtc: avoid using %hhx for Windows

%hhx is introduced in C99 and the MinGW version of gcc on Windows
(as of today v4.8.1) does not support that. Use %2x instead, and
an explicit (unsigned char) cast is needed for the output char
to prevent the unexpected sign extension.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>